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BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI 
BIOTYPES  AND  HYBRIDS 


BY 


GEORGE  HARRISON  SHULL 


WASHINGTON,  D.  C. 
Published  by  the  Carnegie  Institution  of  Washington. 

1909. 


Carnegie  Institution  of  Washington  Publication  No.  112 


Papers  of  the  Station  for  Experimental  Evolution  No.  12 


THE  CORNMAN  PRINTING  COMPANY 
CARLISLE,   PENNSYLVANIA 


BURSA  BURSA- PASTORIS  AND  BURSA  HEEGERI 
BIOTYPES  AND  HYBRIDS. 


Bv  George  Hakrison  Shull. 


INTRODUCTION. 

Darwin  recog'nized  in  the  facts  of  variation  a  key  to  the  riddle  presented 
by  the  miiltiformity  and  many  obvious  interrelations  of  all  living"  things. 
Since  the  appearance  of  the  Orig-in  of  Species  the  observation  and  discus- 
sion of  variations  have  assumed  a  dominant  place  in  biolog"y,  and  a  serious 
conflict  has  recently  developed  reg-arding"  the  interpretation  of  the  observed 
fact^.  It  is  now  g-enerally  recognized  that  this  conflict  can  be  brought 
to  a  termination  only  through  the  application  of  experimental  methods. 
Inspection  alone  can  not  decide  the  question  as  to  how  an  observed  varia- 
tion originated  and  what  bearing  it  may  have  on  the  future  of  the  race  in 
which  it  occurs.  Studies  in  the  museum  and  in  the  field  only  discover  the 
fact,  and  to  some  extent  the  rang-e,  of  variation  occurring  under  a  more  or 
less  limited  and  inadequately  known  rang-e  of  conditions,  and  can  not  cer- 
tainly determine  its  cause  or  causes  ;  neither  can  these  means  supply  more 
than  a  suggestion  based  upon  insecure  inference  as  to  the  hereditary  nature 
of  any  variation.  The  causes  of  variation  can  be  determined  only  by 
subjecting-  eqiiivalent  material  to  different  controlled  conditions,  and  their 
hereditary  relations  can  be  learned  only  through  the  conduct  of  pedigree- 
cultures. 

We  already  know,  as  a  result  of  experimental  work  in  these  directions, 
that  variations  are  of  fundamentally  different  types,  having"  different  causes 
and  obeying  different  laws  of  development  and  heredity.  A  knowledg-e  of 
these  facts  impresses  an  important  principle,  namely,  that  the  rang"e  of 
applicability  of  any  conclusion  reached  by  the  investigation  of  one  class  of 
material  or  one  characteristic  can  be  determined  only  by  similar  experiments 
with  other  material  and  other  characteristics. 

This  has  been  one  of  the  guiding  principles  in  the  institution  of  the  ex- 
perimental work  at  the  Station  for  Experimental  Evolution  of  the  Carnegie 
Institution  of  Washington.  Numerous  species  of  plants  and  animals  of 
quite  wide  relationships  have  been  brought  under  observation,  and  it  may 
be  expected  that  each  will  in  time  g"ive  material  assistance  in  determining 
to  what  extent  principles  and  hy]5otheses  now  available  have  general  validit}', 
and  will  also  lead  to  the  discovery  of  such  new  ]irincii^les,  or  such  modifi- 

3 


Library 
H.  C,  State  CoUesr©  g ^  )54*? 


4  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

cations  of  the  old  ones,  as  may  be  necessary  for  the  interpretation  of  the 
observed  facts  of  evolution.  Among  the  species  now  being  studied ,  one  that 
has  very  early  yielded  results  of  interest  in  this  connection  is  the  common 
shepherd' s-purse  of  our  dooryards,  Bursa  bursa-pastoris  (L.)  Britton. 

Bursa  {Capsella)  bursa-pastoris  is  known  to  the  taxonomist  as  an  exceed- 
ingly variable  species,  which  seems  to  have  been  brought  to  this  country  from 
Europe,  but  which  is  now  naturalized  and  almost  universally  distributed 
throughout  the  North  Temperate  zone.  So  strikingly  different,  qualita- 
tively, were  the  characters  of  different  individuals  observed  growing  side 
by  side  in  nature  that  it  was  difficult  to  believe  that  they  all  belonged  to  a 
single  series  of  fluctviating'  variations,  and  when  the  opportunity  offered 
to  make  pedigree-cultures  this  species  at  once  suggested  itself  as  favorable 
material.  This  thought  was  based  not  alone  upon  its  apparent  polymorph- 
ism, but  also  upon  its  hardiness,  ease  of  culture,  and  the  impossibility  of 
its  having  been  subjected  to  any  of  the  artificial  conditions  of  isolation, 
crossing,  etc.,  which  are  usually  thought  to  render  plants  of  economic 
value  unfit  to  give  information  regarding  the  behavior  of  plants  in  nature. 

No  one  has  ever  attempted  to  "improve"  the  shepherd's-purse,  and 
although  its  rapid  extension  over  its  present  great  range  is  undoubtedly 
dependent  upon  the  agency  of  man,  both  in  supplying  suitable  habitats  and 
more  directly  in  the  transportation  of  the  seeds,  yet  in  performing  these 
operations  his  work  has  been  wholly  unintentional,  and  he  is  therefore  to 
be  classed  with  the  other  accidental  agents  of  nature,  thus  leaving  to  Bursa 
the  solution  of  its  problems  of  maintenance,  extension  of  rang'e,  and  evo- 
lutionary progress,  under  conditions  which  are  fundamentally  like  those 
that  mvist  be  met  by  any  other  species  in  a  state  of  nature. 

These  cultures  were  begim  in  April,  1905,  and  were  continued  until  the 
spring  of  1907,  when  they  were  temporarily  abandoned  because  other  in- 
vestigations necessitated  my  absence  from  the  Station  for  Experimental 
Evolution  during  rather  extended  periods,  and  thus  made  it  impossible  to 
continue  the  work  advantageously  on  a  species  like  Bursa,  which  shows  no 
dependence  upon  the  seasons,  but  blooms  and  fruits  whenever  external 
conditions  are  such  as  to  make  its  development  possible.  It  is  hoped  that 
these  studies  on  Bursa  may  be  continued  in  the  not  distant  future,  but  as 
some  of  the  conclusions  arrived  at  have  been  already  presented  before  sev- 
eral scientific  bodies,*  it  seems  desirable  to  publish  a  more  comprehensive 
account  of  the  work  than  has  been  done  up  to  this  time,  even  though  the 
evidence  is  in  many  places  more  or  less  fragmentary. 

When  undertaking  such  cultures  with  any  new  class  of  material  much 
that  is  necessary  for  the  most  satisfactory  and  economic  conduct  of  exper- 

*Sections  F  and  G,  A.  A.  A.  S.,  New  York,  December,  1906;  Seventh  International 
Zoological  Congress,  Boston,  August,  1907;  Botanical  Society  of  America,  Chicago, 
January,  1908. 


BIOTYPES  AND  HYBRIDS.  5 

iments  must  be  learned  by  experience,  for,  as  said  before,  mere  inspection 
of  an  individual  or  of  a  single  g-eneration  can  not  distingiiish  between  im- 
portant and  unimportant  variations  until  the  pedig-rees  themselves  furnish 
solutions  to  the  question.  For  this  reason  there  must  be  in  the  beginning 
a  much  more  minute  analysis  of  the  material  than  is  later  found  necessary 
or  desirable.  Most  of  the  cultures  which  I  have  made  thus  far  have  been 
directed  toward  determining-  what  variations  are  of  the  fluctuating  kind  and 
what  are  ftilly  transmissible  to  the  offspring. 

In  one  group  I  have  now  had  large  numbers  of  plants  of  the  fourth 
pedigreed  generation  and  a  few  of  the  fifth  under  observation,  and  in  a 
number  of  other  cases  the  third  generation  has  been  extensively  grown.  I 
find  that  while  certain  variations  which  were  selected  disappear  in  the  first 
or  second  generation,  others  remain  constant,  easily  recognized  differen- 
tiating marks  which,  except  in  one  form,  show  no  transgression  of  the  char- 
acteristic features  of  any  other  form  studied.  These  forms  are,  therefore, 
distinct  elementary  species,  or  biotypes,  each  characterized  by  certain  con- 
stant features  and  each  with  its  own  normal  range  of  fluctuating  variability. 

The  systematist  has  not  yet  decided  what  treatment  to  give  to  elementary 
species,  and  any  nomenclatorial  scheme  must  be  regarded,  therefore,  as 
purely  tentative  ;  but  utility  can  not  wait  for  concerted  action  on  the  part 
of  taxonomists  in  devising  a  suitable  systematic  designation  of  elementary 
species,  and  I  have  therefore  for  the  sake  of  convenience  assigned  to  these 
elementary  forms  of  Bursa  simple  names  which  can  be  attached  to  the 
accepted  specific  name  to  form  a  trinomial.  I  was  at  first  inclined  to  use 
binomial  names  which  would  leave  the  Linnean  specific  name,  bursa- 
pastoris,  as  the  valid  name  of  the  aggregation  of  elementary  forms  having 
the  same  general  habit  and  the  triangular  or  obcordate  capsules.  It  might, 
then,  be  looked  upon  as  a  superspecific  name  which  would  remain  just  as 
useful  in  the  everyday  conversation  and  experience  of  the  botanist  as  when 
the  aggregation  for  which  it  stands  was  believed  to  be  a  unit. 

The  fact  that  corresponding  series  of  elementary  species  or  biotypes  may 
occur  in  different  related  species,  as  will  be  shown  later  in  the  discussion 
of  the  hccgeri  hybrids,  makes  the  trinomial  much  to  be  preferred,  for  cor- 
responding forms  may  then  be  given  the  same  name  without  confusion. 
Thus  Bursa  bursa-pastoris  lieteris  and  Bursa  heegeri  heteris  may  be  used  to 
denote  two  forms  which  are  alike  in  rosette -characters  but  different  in  the 
capsule-character,  the  latter  character  being  accepted  as  of  specific  value. 

That  Bursa  bursa-pastoris  is  a  composite  species  was  first  given  public 
recognition  by  Lotsy  (1906)  about  a  year  after  my  cultures  were  begun, 
and  his  statement  did  not  come  to  my  notice  until  after  I  had  presented 
my  first  account  before  the  American  Association  for  the  Advancement  of 
Science  at  its  New  York  meeting,  December,  1906.  His  statement  is  very 
brief,  and  the  chief   interest  of   his  account  consists  in  the  photographs 


6  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  '. 

which  show  samples  of  three  families  which  bred  triie  to  rather  strikinoflj' 
distinct  characters  in  his  cultures.  To  these  several  forms  he  assi.arned 
trinomial  names,  thoug-h  no  adequate  description  is  .sriven.  No  statement  is 
made  as  to  the  extent  of  the  cultures  upon  which  their  standin.sr  as  perma- 
nent biotypes  is  based,  nor  is  there  any  statement  as  to  the  extent  and  nature 
of  the  fluctuating-  variability  of  each  of  the  forms  figured.  Until  attention 
is  given  to  such  matters  by  the  student  of  pedigTee-cultures  it  will  be  im- 
possible to  properly  estimate  the  security  of  conclusions  drawn,  or  to  rec- 
og-nize  with  any  certainty  the  identity  of  biotypes  discovered  by  different 
investigators. 

To  obviate  these  difficulties,  I  wrote  to  Dr.  Lotsy  for  pedigreed  seeds  of 
his  several  types,  in  order  that  they  might  be  grown  beside  my  own  for  the 
purpose  of  testing  their  identity,  but  received  the  reply  that  none  were 
available.  The  questions  as  to  whether  Lotsy 's  names  are  therefore  to  be 
considered  nomina  Jiuda  and  whether  I  am  justified  in  assigning  new  names 
to  the  biotypes  to  be  discussed  below,  which  may  or  may  not  be  identical 
with  one  or  more  of  Lotsy' s  named  elementary  species,  I  leave  to  the  tax- 
onomist  to  decide.  Indeed,  in  adopting  the  names  used  in  this  paper  I  have 
been  governed  entirely  by  the  demands  of  present  utility  and  not  by  any 
thought  that  these  names  will  be  accejDted  by  taxonomists  as  having  proper 
standing  in  the  nomenclatorial  system  now  recognized  by  them. 

Still  more  recently,  Almquist  (1907)  has  published  over  70  named  "  ele- 
mentary species"  of  B.  bursa-pastoris,  but  these  are  described  in  rather 
general  terms,  and  the  cultures  were  conducted  in  the  open  garden  un- 
guarded, usually  for  but  2  to  4  generations.  Almquist  states  (p.  5)  that 
after  this  length  of  time  the  cultures  had  either  died  out  or  were  no  longer 
pure.  He  assumes  that  this  "  loss  of  purity  "  is  due  to  vicinism  with  the 
other  elementary  species  growing  on  neighboring  plots,  and  some  of  it 
doubtless  was.  In  my  own  cultures  I  have  observed  that  some  of  the  dif- 
ferentiating characters  used  by  Almquist  disappear  upon  continued  breed- 
ing, even  when  vicinism  is  carefully  excluded.  It  seems  that  members  of 
the  same  biotype  coming  from  different  habitats  may  retain  certain  fluctu- 
ating differences,  such  as  differences  in  texture  of  leaves,  degree  of  redness 
of  veins,  degree  of  epinastic  growth  in  the  petioles,  etc.,  for  several  gener- 
ations of  uniform  treatment,  but  finally  merge  into  identical  forms,  even 
when  crossing  with  other  biotypes  is  entirely  precluded  by  careful  guarding. 

While  I  can  not  say  with  certainty  that  any  of  Almquist's  "elementary 
species"  are  not  distinct  and  permanent  types,  my  experience  indicates 
emphatically  that  he  has  not  demonstrated  that  they  are.  I  think  it  very 
probable  that  a  number  of  the  elementary  species  figured  and  described  by 
him  will  not  stand  the  test  of  longer  culture  under  more  carefully  controlled 
conditions.  However  this  may  be,  it  seems  evident  that  the  number  of 
elementary  species  of  Bursa  may  be  considerable.     At  the  time  of  the  first 


BIOTYPES  AND  HYBRIDS.  7 

presentation  of  my  studies  on  Bursa  before  the  American  Association  for 
the  Advancement  of  Science  (December,  1906),  I  was  prett}^  sure  that  I 
had  demonstrated  the  existence  of  11  biotypes,  but  the  disappearance  of  one 
or  two  of  these  since,  led  me  to  reduce  the  number  to  4,  as  imbHshed  in  the 
extract  from  that  pai:)er  (Shull,  1907). 

These  four  forms,  whose  distinctness  and  permanence  I  have  demon- 
strated beyond  a  possible  ([uestion,  liave  been  studied  in  their  hybrid  com- 
binations, and  it  is  the  cultures  oi  these  and  their  hybrids  with  which  the 
present  paper  will  mainly  deal,  though  some  features  of  other  cultures  will 
be  discussed. 

It  is  doubtful  whether  any  bit  of  technique  that  has  been  recently 
added  to  the  tools  with  which  the  biolog-ist  may  operate  in  unlocking-  the 
mysteries  of  protoplasmic  organization  is  of  so  far-reaching  importance 
as  the  process  of  hybridization.  I  say  has  been  recently  added  because, 
although  the  art  of  producing  hybrids  is  very  old  among  the  breeders  of 
animals,  and  nearly  200  years  old  among  gardeners,  little  of  scientific  value 
could  be  secured  by  means  of  hybridization  until  some  of  the  fundamental 
laws  involved  in  the  process  were  recognized,  and  it  is  only  within  the  last 
eight  years  that  biologists  have  gained  sufficient  insight  into  the  behavior 
of  hybrids  to  give  interpretations  of  the  results  of  hybridization  any  value 
as  indications  of  protoplasmic  structure  and  behavior. 

Two  general  types  of  hybrids  are  readily  recognizable,  namely,  the  con- 
stant and  the  splitting.  The  former  may  or  may  not  give  indication  of  the 
characteristics  of  their  parents,  being  usually,  but  not  always,  intermediate 
between  the  parents  in  most  of  their  characters.  The  splitting  hybrid 
always  indicates  by  its  offspring  what  were  the  characteristics  of  its  im- 
mediate ancestors.  It  is  the  latter  type  of  hybrid  which  is  of  the  greatest 
usefulness  in  giving  insight  into  the  stinicture  of  the  germ-plasm.  Hybrid- 
ization in  such  cases  does  not  only  serve  to  unite  in  the  same  individual  all 
of  the  characteristics  of  both  parents,  but  as  successive  generations  are 
followed,  it  results  in  a  complete  analysis  of  all  the  points  of  difference 
existing  in  the  two  biotypes  between  which  the  cross  was  made. 

The  conduct  of  such  analysis  by  hybridization  is  particularly  simple  in 
the  case  of  plants  which,  Hke  Bursa,  reaclily  self-fertilize,  because  once  the 
first-generation  hybrid  is  secured  the  process  of  analysis  goes  on  genera- 
tion after  generation,  until  all  the  allelomorphic  dilferences  of  the  parents 
are  made  manifest  by  being  separated  in  a  pure  state  in  different  individuals. 

In  some  respects  even  more  satisfactory  evidence  of  the  elementary  char- 
acter of  two  forms  is  to  be  derived  from  their  behavior  when  crossed  than 
from  their  conduct  in  straight  pedigrees.  If,  for  instance,  two  forms  sup- 
posed to  be  elementary  to  each  other  should  be,  instead,  merely  extreme 
fluctuants  of  a  single  biotype,  their  cross-bred  progeny  would  show  a  fluc- 
tuating series,  including  perhaps  the  two  parental  extremes,  but  could  hardly 


8  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

be  expected  to  produce  a  constant  intermediate  progeny,  nor  one  which 
would  show  the  two  parental  conditions  in  an  expected  Mendelian  ratio. 
Occasionally  in  the  course  of  pedigree-cultures  some  slight  difference  in 
treatment  will  produce  a  modification  in  some  progeny,  which  may  lead  to 
doubt  as  to  its  classification,  and  perhaps  even  after  several  generations 
this  doubt  will  not  be  wholly  removed.  In  such  cases  hybridization  is 
almost  certain  to  give  decisive  results,  as  there  often  appears  to  be  a  more 
characteristic  development  of  morphological  features  in  the  hybrids  than  in 
the  pure-bred  strains.  This  may  be  due  to  the  fact  that  the  allelomorphs 
are  brought  into  different  relations  with  other  allelomorphs  in  the  recom- 
binations which  take  place  in  the  second  and  later  generations,  thus  elimi- 
nating any  inhibiting  or  modifying  influence  which  may  have  resulted  from 
the  constant  presence  of  some  particular  allelomon^h  or  combination  of 
allelomorphs,  or  of  some  permanent  condition  of  the  cytoplasm,  in  the  pure 
strain.  This  elimination  of  modifying  factors  would  allow  a  more  accurate 
definition  of  the  unit-characters  involved.  Or,  on  the  other  hand,  the  better 
development  of  characters  in  the  hybrids  may  be  due  to  the  fact,  long 
recognized,  that  heterozygosis  produces  a  stiinulation  which  increases 
vegetative  vigor,  and  this  should  have  the  same  effect  as  good  cultural 
conditions  in  bringing  all  characters  to  their  full  tjq^ical  development. 

MATERIAL  AND  METHODS. 

For  the  beginning  of  the  cultures,  seeds  were  secured  from  a  number  of 
more  or  less  strikingly  different  individuals  growing  in  the  vicinity  of  Cold 
Spring  Harbor,  Long  Island;  Chicago,  Illinois;  New  Carlisle,  Ohio;  Man- 
hattan, Kansas;  and  Tucson,  Arizona.  Leaves  of  each  of  the  plants  chosen 
as  parents  for  the  cultures  were  carefully  preserved,  in  order  that  the  con- 
ditions of  the  offspring  might  be  compared  with  those  of  the  parent  and, 
in  each  pedigreed  family  raised  since,  care  has  been  taken  to  preserve  in 
the  herbarium  samples  of  every  variation  of  sufficient  magnitude  to  strike 
the  eye. 

From  20  lots  of  seeds  of  Bursa  biirsa-pastoris  brought  in  from  nature 
and  1  lot  of  B.  heegcri  received  hx  Dr.  MacDougal  from  Professor  Solms- 
Laubach,  I  have  now  had  under  observation  about  200  pedigreed  families, 
including  something  over  26,900  individuals  of  Bursa  bursa-pastoris,  2  small 
families  of  Bursa  heegeri,  and  5  families  representing  reciprocal  crosses 
between  these  two  species  and  involving  over  2,500  individuals. 

All  of  my  cultures  have  been  carried  on  imder  glass,  the  first  7  months 
in  the  sk>--lighted  room  of  the  laboratory  at  the  Station  for  Experimental 
Evolution,  and  since  that  time,  /.  e.,  after  December  7,  1905,  in  the  glass 
propagating-house  at  the  same  place.  It  was  found  that  the  light  was  too 
dim  in  the  former  room  to  bring  out  the  characteristic  features  of  the  plants 
and  to  keep  them  in  a  state  of  vigorous  health.     The  leaves  became  more 


BIOTYPES  AND  HYBRIDS.  9 

elong'ated  than  is  normal,  and  the  lobing'  more  shallow  and  therefore  less 
characteristic,  so  that  it  was  less  easy  to  estimate  the  uniformity  and  dis- 
tinctness of  the  several  pedigrees  than  when  they  were  grown  under  more 
favorable  conditions.  Even  specimens  belonging  to  the  most  deeply  and 
distinctly  lobed  families,  when  grown  in  the  darker  portions  of  the  room, 
retained  their  unlobed,  juvenile  tjq^e  of  leaf  throughout  life,  sending  up  a 
weak  flower-stem  from  the  juvenile  rosette,  and  in  such  cases  the  relation- 
ships could  not  be  recognized,  since  the  early  leaves  of  all  the  forms  studied 
are  very  similar.  The  same  difficulty  was  also  experienced  in  the  propa- 
g'ating-house  when  the  inembers  of  certain  families  were  allowed  to  remain 
too  long  crowded  in  the  seed-pans .  The  offspring  of  these  juvenile  plants 
have  not  been  extensively  studied ,  but  from  several  families  evidence  has 
accumulated  which  indicates  that  when  again  given  favorable  conditions 
the  offspring  of  these  characterless  plants  return  to  the  characters  of  the 
family  from  which  they  sprang. 

In  all  cases  the  seeds  have  been  sown  in  soil  sterilized  in  an  autoclave 
in  the  manner  usually  adopted  by  students  of  pedigree-cultures,  and  the 
efficiency  of  the  method  is  inferable  from  the  fact  that  in  all  these  cultures 
only  3  seedlings  occurred  whose  origin  was  unknown.  These  unexpected 
seedlings  were  an  Oxa/is,  a  Molhigo,  and  an  Erechtites,  and  it  does  not  seem 
likely  that  any  of  these  withstood  the  long^-continued  high  temperature  of 
the  autoclave,  but  rather  that  they  were  blown  through  the  ventilators  of 
the  propagating'-house  in  a  heavy  wind-storm. 

It  was  soon  demonstrated  that  Bursa  has  many  features  which  make  it 
advantageous  material  from  a  technical  point  of  view  for  pedigree-culture 
work.  The  habit  of  the  plant,  consisting  as  it  does  of  a  moderately  lax 
rosette  and  nearly  naked,  erect  flower-stem,  allows  the  preservation  of  the 
rosettes  as  herbarium  specimens  whose  characters  are  almost  as  easily 
studied  as  are  those  of  the  living  plants,  and  the  inflorescence  may  be  cov- 
ered with  paraffin-paper  bags  to  prevent  chance  crosses  with  other  speci- 
mens without  appreciably  interfering  with  the  photosynthetic  work  upon 
which  the  healthy  development  of  the  plant  depends. 

The  small  size  of  the  plants  makes  it  possible  to  raise  them  to  maturity 
in  3-inch  pots  in  many  of  the  forms,  though  it  is  usually  found  advanta- 
geous to  repot  to  4-inch  pots  those  which  it  is  desired  to  keep  for  seed. 
This  quality  allows  a  large  number  of  specimens  to  be  raised  in  a  small 
compass. 

In  most  instances  the  life-cycle  is  short,  requiring  only  3  to  4  months 
between  the  sowing  of  the  seed  and  the  gathering  of  the  rij^e  seed  of  the 
earliest  matured  individuals.  In  several  of  the  forms  of  B.  bursa-pastoris, 
and  in  B.  heegeri,  however,  8  to  9  months  were  needed.  The  seeds  ger- 
minate in  5  to  8  days  without  a  period  of  rest,  thus  making  it  possible  to 
accumulate  data  from  a  number  of  successive  generations  in  a  short  time. 


10 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI 


The  flowers  are  adapted  to  both  cross-  and  self-fertilization,  all  the  evi- 
dence now  at  hand  indicating-  that  the  latter  method  is  normally  by  far  the 
more  efficient.  Leaving  out  of  account  2  of  the  21  original  cultures  which 
showed  by  their  constitution  that  their  parents  were  hybrids,  less  than  1 
per  cent  of  the  plants  raised  from  seed  collected  in  nature  showed  evidence 
of  being  the  result  of  cross-fertilization  between  different  biotypes .  Crosses 
between  different  flowers  of  the  same  plant  and  between  different  indi- 
viduals of  the  same  biotype  may  take  place  somewhat  more  frequently  than 
crosses  between  the  flowers  of  plants  belonging  to  different  biotypes, 
though  nothing  in  the  resultant  offspring  gives  any  clue  to  the  frequency 
of  such  crosses. 


Fig.  1. — Enlarged  sections  of  buds  and  flower  of  Bu?-sa  bursa pastoris,  show- 
ing three  stages  in  anthesis.  A.  Exposure  of  the  stigmatic  surface  for  the 
reception  of  foreian  pollen.  B.  Anthers  opening  in  contact  with  the  stigma, 
thus  insuring  self-pollination.  C.  The  flower  fully  open,  allowing  the  access 
of  the  visiting  insects  to  the  pollen.     All  magnified  20  diameters. 

The  conditions  which  favor  cross-fertilization  are  :  (a)  Slight  prote- 
rogyn}',  which  allows  the  stigina  to  receive  foreign  pollen  some  hours  before 
the  anthers  of  the  same  flowers  dehisce.  The  distal  portion  of  the  globular 
or  disk-shaped  stigma  is  exposed  between  the  tips  of  the  unopened  sepals  and 
petals  (fig.  1 ,  a)  .  Qy)  Although  the  fully  developed  inflorescence  is  a  typical 
raceme,  the  flowers  and  buds  are  arranged  in  a  nearly  flat-topped  corymb 
having  the  flowers  at  the  circumference,  giving  this  part  of  the  inflores- 
cence a  condition  quite  analog'ous  to  that  of  the  head  of  the  Compositae,  in 
which  the  whole  inflorescence  appears  to  act  the  part  of  a  single  flower  in 
the  attraction  of  insects.  Small  insects,  particularh-  flies  and  small  bees, 
visit  the  flowers  freely.  These  rest  upon  the  top  of  the  inflorescence  as  a 
whole  while  visiting  the  several  individual  open  flowers  about  the  circum- 
ference. In  these  fully  open  flowers  forming  the  exterior  circle  of  the  inflo- 
rescence the  anthers  have  opened,  while  in  the  second  circle  the  summits 
of  the  stigmas  are  exposed  on  the  same  general  level  as  the  rest  of  the 
corymb.     As  the  insect  walks  about  over  the  top  of  the  inflorescence,  the 


BIOTYPES  AND  HYBRID.S.  11 

chances  appear  favorable  for  the  lodg-ment  of  i)ollen  from  the  outer  circle 
of  flowers,  or  from  the  flowers  of  other  plants,  upon  the  exposed  stigma-tops 
of  the  second  circle. 

Adaptation  to  self-fertilization  is  found  in  the  facts:  («)  that  the  stigma 
is  also  receptive  on  its  under  (proximal)  surface  as  well  as  on  the  upper 
(distal)  surface,  and  (/;)  that  the  anthers  dehisce  while  they  are  still  held 
in  contact  with  this  under  surface  of  the  stigma  by  the  erect  segments 
of  the  perianth  (fig.  1,  b).  vSoon  after  the  anthers  open  the  petals  begin 
to  spread,  thus  exposing-  the  pollen  to  be  carried  away  to  other  flowers  by 
visiting  insects  (fig.  1,  c). 

At  the  beginning  of  these  experiments  paraffin-paper  bags  of  suitable 
size  were  not  available,  and  as  insects  were  not  abundant  in  the  room  used 
for  the  cultures,  the  only  precaution  taken  to  guard  against  cross-pollination 
was  to  set  the  flowering  specimens  intended  as  seed-plants  somewhat  apart 
from  each  other  and  from  other  flowering  specimens.  As  will  appear 
later,  a  few  individuals,  of  unexpected  character,  are  doubtless  to  be  attrib- 
uted to  this  unguarded  condition  of  the  earlier  cultures,  but  it  will  also  be 
seen  that  the  percentage  of  such  chance  crosses  is  extremely  small.  As 
soon  as  possible,  suitable  paper  bags  were  secured,  and  since  then  the  cul- 
tures, with  few  exceptions,  have  been  carefully  guarded. 

Although  crossing  among  the  unguarded  cultures  in  the  greenhouse  has 
been  of  rare  occurrence,  it  was  evidently  more  frequent  in  the  material 
secured  from  nature.  Of  the  21  original  cultures  2  proved  to  be  of  hybrid 
origin,  while  a  fraction  of  1  per  cent  of  the  rest  indicated  by  their  atypic 
condition  that  they  were  probably  the  result  of  cross-pollination.  Only 
when  the  pollen  comes  from  some  form  which  is  dominant  to  the  pistil- 
parent  is  the  fact  that  a  cross  has  taken  place  obvious  in  the  first  genera- 
tion. It  seems  fair  to  assume  that  on  the  average  as  many  crosses  take 
place  with  a  recessive  pollen-parent  as  with  a  dominant,  and  this  assump- 
tion would  require  that  hybridizations  occur  with  twice  the  frequency  with 
which  they  become  obvious  in  the  F,  offspring.  On  this  basis  the  fre- 
quency of  cross-pollination  between  different  biotypes  of  Bursa  in  nature, 
as  indicated  by  these  cultures,  is  about  1  to  65  as  compared  with  the  fre- 
quency of  self-pollination  and  crosses  between  flowers  of  the  same  plant 
or  between  plants  of  the  same  biotypc.  Of  course  this  ratio  is  based  upon 
a  very  limited  number  of  specimens  and  can  be  expected  to  vary  greatly 
in  different  lots  of  material  of  the  same  magnitude,  but  at  least  the  great 
preponderance  of  self-fertilization  may  be  safely  inferred  in  this  species 
when  in  a  state  of  nature. 

These  characteristics  of  Bursa  which  make  the  production  of  pure  self- 
fertilized  lines  easy  are  opposed  to  the  ease  with  which  cross-fertilization 
may  be  controlled.  The  fact  that  self-fertilization  takes  place  before  the 
petals  spread  makes  it  necessary  to  carefully  remove  the  stamens  about  a 


12  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

day  before  the  buds  open.  As  the  buds  are  at  that  time  quite  small,  the 
technique  of  cross-pollination  is  somewhat  delicate.  However,  with  a 
needle-pointed  forceps  the  calyx,  corolla,  and  stamens  may  be  readily  cut 
away  from  about  the  young"  pistil,  and  the  eye  and  hand  soon  become  so 
skilled  that  the  work  can  be  done  swiftly  and  with  as  much  accuracy  as 
may  be  attained  in  the  castration  of  a  lily. 

PURE  CULTURES  OF  BURSA  BURSA-PASTORIS. 

The  four  biotypes  of  Bursa  bursa-pastoris  with  which  this  paper  mainly 
deals  are  distinguished  from  each  other  by  certain  characteristic  lobings  of 
the  leaves.  For  convenience  I  have  assigrned  to  them  the  names  Bursa 
bursa-pastoris  heteris,  B.  bp.  tenuis,  B.  bp.  rhomboidca,  and  B.  bp.  simplex. 
It  has  been  impossible  to  determine  which  of  Almquist's  "elementary 
species"  agree  with  these,  but  it  is  almost  certain  that  he  has  assigned 
names  to  several  fluctuations  of  the  same  biotype.  If  this  is  true  my  four 
types  are  more  inclusive  than  his  and  under  my  names  will  need  to  be 
grouped  a  number  of  forms  which  he  has  considered  distinct  elementary 
species. 

Bursa  bursa-pastoris  heteris  n.  sp.  element. 

Plants  belonging  to  this  biotype  have  the  leaves  divided  to  the  midrib, 
the  terminal  lobe  being  usually  separated  from  the  nearest  lateral  lobes  by 
deep,  clean-cut  incisions.  The  lateral  lobes  consist  essentially  of  two  fea- 
tures— an  elongated,  attenuate  portion  which  I  call  the  '  'primary  lobe, ' '  and 
a  more  or  less  rounded  or  angular  portion  which  forms  a  ' '  secondary  lobe  ' ' 
in  the  distal  axil  of  the  primary  lobe  (fig.  2).  As  in  the  characterizations 
of  all  the  following  forms,  this  description  refers  to  the  climax-leaves  of  the 
rosettes  in  properly  grown  specimens,  since  the  juvenile  leaves  of  all  the 
elementary  species  of  Bursa  are  entirely  unlobed,  and  starved  or  crowded 
specimens  of  all  the  forms  may  reach  maturity  with  only  juvenile  leaves, 
as  already  stated.  Several  of  Almquist's  recently  described  forms  would 
obviously  range  themselves  under  this  description,  though  it  is  always  pos- 
sible, of  course,  that  forms  which  possess  the  described  characteristics  may 
have  still  other  permanent  characteristics  which  would  render  them  distinct 
elementary  species. 

The  first  of  the  forms  recognized  by  Almquist  which  can  certainly  be 
placed  here  is  his  Capsella  bp.  rubella  (see  his  figs.  5  and  6).  His  C.  bp. 
angustiloba  (his  figs.  56  and  57)  represents  the  most  pronounced  develop- 
ment of  this  type,  and  others  which  probably  belong  here  are  reiiteri  (fig. 
11),  grandiflora  (figs.  12  and  13),  hiemalis  (fig.  20),  grossa  (fig.  22),  au- 
tiimnalis  (fig.  24),  segetum  (fig.  28),  wittrockii  (fig.  44),  rhombea  (fig.  48), 
rhombella  (fig.  53),  ellipsoidea  (fig.  55),  2iXvdifucorum   (fig.  60). 

My  first  pure-bred  family  of  B.  bp.  heteris  (pedigree-number  040.3)  was 
grown  from  seeds  collected  by  J.  Marion  Shull  at  Edgewood,  New  Carlisle, 


BIOTYPES  AND  HYBRIDS. 


13 


Fig.  2. — Bursa  bursa-pastoris  heteris.     A  typical  specimen  of  my  first  pure  cul- 
ture of  this  biotype. 

Fig.  3. — Bursa  bursa-pastoris  heteris.    Climax  leaves  of  the  rosette  of  a  speci- 
men growing  in  nature  in  Jackson  Park,  Chicago. 


14  BURSA  BURSA-PASTORIvS  AND  BURSA  HEEGERI  : 

Ohio,  May  28  and  June  2,  1905.  The  cHmax  leaves  of  the  parent  were  of 
the  most  pronounced  heteris  form.  From  seeds  sown  June  26,  1905,  39 
specimens  were  grown,  all  of  which  agreed  with  the  parent  and  with  each 
other  in  the  character  of  lobes  as  described,  though  there  was  some  fluctu- 
ation in  general  aspect  due  to  difference  in  elongation  of  the  rachis  by 
means  of  which  the  lobes  were  more  or  less  crowded,  some  variation  in  the 
attenuation  of  the  primary  lobes,  in  the  degree  of  lobation  of  the  upper 
rosette  leaves,  in  the  angle  which  the  rosette  leaves  made  with  the  hori- 
zontal, and  in  the  prominence  of  the  rounded  secondary  lobe.  This  sec- 
ondary lobe  was  characteristic,  however,  and  in  not  a  single  individual  was 
it  absent  in  the  climax  leaves  of  the  rosette. 

The  significance  of  several  of  these  variations  was  tested  by  using  their 
best  representatives  as  parents  of  a  second  generation.  From  3  of  these 
(053.30,  053.31,  053.32)  have  been  raised  839  specimens,  all  but  the 
54  offspring  of  one  individual  being  strictly  like  the  grandparental  and 
parental  form.  The  one  aberrant  family  (053.30),  the  parent  of  which  had 
the  upper  leaves  of  the  rosette  less  deeply  lobed  than  usual,  showed  com- 
plete suppression  of  lobes  in  5  individuals,  and  in  the  remaining  Z^  which 
were  noted  the  primary  lobes  were  broader  than  usual  and  the  secondary 
lobes  less  prominent.  It  is  desirable  to  test  the  possibility  that  these  char- 
acteristic secondary  lobes  may  be  rendered  completely  latent  by  selection, 
and,  if  this  is  possible,  to  discover  by  what  means  they  may  be  again  made 
manifest. 

At  least  two  other  pure-bred  cultures  coming  in  from  nature  belong  to 
B.  bp.  heteris,  but  were  considered  for  two  generations  to  represent  two 
very  distinct  elementar}^  forms.  It  has  been  the  striking  modifications  ob- 
served in  these  two  pedigrees  which  have  led  me  to  suspend  judgment  on 
all  my  cultures  except  those  whose  behavior  in  hybrid  combinations  has 
left  no  possible  question  as  to  their  distinctness  and  permanence. 

The  basis  for  the  assumption  that  these  two  pedigrees  represented  dis- 
tinct elementary  species  was  the  facts  that  the  aspect  of  each  of  these  fam- 
ilies was  very  different  from  that  of  any  other  culture  and  that  there  was 
great  uniformity  among  the  individuals  belonging  to  either  single  family, 
/.  e. ,  while  there  was  a  strong  break  between  the  families  there  was  almost 
no  variation  within  the  family.  This  was  presumably  the  basis  of  Alm- 
(juist's  estimates  as  to  the  distinctness  of  his  forms,  and  the  following  ac- 
count of  these  two  families  supports  my  attitude  of  doubt  as  to  the  soundness 
of  his  results. 

040.15:  This  specimen  was  found  growing  in  Jackson  Park,  Chicago, 
by  Charles  A.  ShuU,  who  collected  the  seeds  in  the  summer  of  1905.  Climax 
leaves  taken  from  the  rosette  show  only  a  slight  development  of  the  heteris 
characters  as  described  above  (fig.  3  ) .  Though  the  sinuses  reached  the  mid- 
rib, the  primary  lobes  were  not  sharply  attenuate,  and  the  secondary  lobes, 


BIOTYPES  AND  HYBRIDS. 


15 


w 


y. 


^\ 


Fig.  4.  Fig.  5. 

Fig.  \.— Bursa  bursapastoris  heteris.  Typical  specimen  from  offspring  of  plant 
shown  in  fig.  3. 

Fig.  5. — Bursa  bursa-pastoris  heteris.  Offspring  of  sib  of  plant  shown  in  fig.  4. 


16  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

thoiig"h  present,  were  not  strongly  marked.  The  seeds  were  sown  in  the 
propag-ating'-house  December  27,  1905,  and  produced  106  offspring,  all  of 
which  had  a  peculiar  g"rayish  aspect,  owing  to  the  fact  that  the  leaves  were 
freely  dotted  with  small  specks  of  red.  The  rosettes  were  very  lax,  the 
relatively  few  leaves  made  a  rather  wide  angie  with  the  horizontal,  and  the 
leaves,  while  possessing  both  the  primary  and  secondary  lobes,  had  the 
latter  small  and  angular  and  little  more  marked  than  serrations  borne  by 
the  primary  lobe,  which  all  experience  up  to  the  present  time  indicates  are 
of  only  fluctuating  value.  (See  fig.  4.)  All  had  simple  slender  stems 
which  came  to  bloom  in  3  to  4  months  from  the  time  the  seeds  were  sown 
as  compared  with  8  months  required  by  the  above-described  family  of 
typical  B.  bp.  heteris. 

Self-fertilized  seeds  of  two  of  these  plants  (0515.95  and  0515.96)  were 
sown  in  May,  1906,  and  g"ave  uniform  progfenies,  but  having;  an  aspect  quite 
different  from  that  of  the  parent  g"eneration  just  described.  Although  the 
rosettes  in  this  second  generation  were  strongly  ascending  and  were  few- 
leaved,  as  in  the  preceding'  generation,  the  texture  of  the  leaves  was  much 
less  firm,  the  grayish  aspect  was  wholly  lost,  owing  to  the  absence  of  the 
red  specks,  and  secondary  lobing  was  also  much  reduced,  giving-  a  condition 
resembling  the  g^randparent  as  it  grew  in  nature  (cf .  figs .  3  and  5 ) .  The  con- 
ditions in  the  propagating-house  during-  the  development  of  these  families 
were  unsatisfactory,  and  their  g-reat  change  from  the  characteristics  of  the 
preceding-  generation  did  not  shake  my  faith  in  their  distinctness  froin  my 
other  types,  since  within  the  family  there  was  still  great  uniformity,  seem- 
ing thus  to  demonstrate  that  whatever  differences  were  observed  were  due 
to  the  differences  in  environment  during  the  time  of  development  of  these 
two  generations. 

An  average  specimen  from  one  of  these  two  families  was  chosen  as  the 
parent  of  a  third  generation.  The  pollination  of  this  plant  (0695.158)  was 
unguarded,  but  it  was  grown  well  separated  from  all  other  Bursa  cultures. 
The  seeds  were  sown  on  November  1,  1906,  and  produced  213  plants,  all 
of  which  possessed  well-marked  the  characteristics  of  B.  bp.  heteris,  having 
completely  lost  all  the  peculiarities  which  had  led  me  to  believe  that  this 
pedigree  belonged  to  a  distinct  biotype.  (See  fig.  6.)  Other  proofs  that 
040.15  was  a  specimen  of  B.  bp.  heteris  were  derived  from  hybrid  families 
formed  by  crossing  its  offspring  with  plants  belonging  to  other  biotypes. 
Except  in  one  case  only  the  Fj  hybrids  from  these  crosses  have  been 
studied  and  they  will  not  be  considered  in  detail  at  this  time,  but  these  first- 
generation  hybrids  showed  the  characteristics  which  they  should  have  pos- 
sessed if  typical  B.bp.  heteris  had  been  used .  The  one  F^  family  (0693 .  203 ) 
which  has  been  reared  from  these  hybrids  will  be  considered  later.  (See 
p.  42.) 


BIOTYPES  AND  HYBRIDS. 


17 


050.80:  Another  orig-inal  culture  which  now  seems  to  belong:  to  B .  dp. 
heteris  was  the  offspring'  of  a  very  robust  plant  collected  in  a  dooryard  near 
Cold  Spring-  Harbor,  Long-  Island,  April,  1906.  This  plant  was  taken  up 
and  potted  in  the  greenhouse,  where  its  pollination  was  g;uarded.  The 
earlier  leaves  of  this  plant  were  recog'nized  as  resembling-  B.  bp.  heteris y  ex- 
cept in  the  less  sharp  attenuation  of  the  primary  lobes,  but  later  leaves— 
the  climax-leaves — were  larg-e,  and,  in  addition  to  the  secondary  lobes 
characteristic  of  B.  bp.  /leteris,  they  had  somewhat  quadrang-ular  secondary 


Fig.  ^.— Bursa  bursa-pastoris  heteris.     Offspring  of  sib  of  plant  shown  in  fig.  5. 

lobes  in  the  proximal  axils  of  the  primary  lobes,  and  these  square  lobes 
were  sometimes  almost  cut  off  from  the  primary  lobes,  giving"  the  leaf  the 
peculiar  form  usually  described  as  interruptedly  pinnate  (fig-.  7).  The 
seeds  of  this  plant  were  sown  May  23,  1906,  and  produced  352  offspring:. 
These  were  badly  damagfed  by  the  thrips,  81  being:  killed  and  57  so  stunted 
as  to  make  an  estimation  of  their  characters  uncertain.  The  remaining- 
214  formed  a  consistent  group  unlike  its  parent  and  also  unlike  any  other 


18 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI 


of  my  cultures.  Few  .showed  even  a  sugfgestion  of  the  squarish  lobes 
which  were  so  conspicuous  in  the  parent,  thus  indicating-  that  the  strong' 
development  of  that  characteristic  in  the  parent  was  in  all  probability 
merely  a  fluctuation.  These  plants  differed  from  those  considered  typical 
of  B.  dp.  heteris  in  having  the  primary  lobes  of  the  climax-leaves  oblong 
and  blunt,  not  attenuate.  However,  in  some  specimens  the  later  leaves 
showed  the  attenuate  lobes  of  typical  B.  hp.  heteris,  and  this  fact  leaves 
little  doubt  that  another  generation  would  have  completely  demonstrated 
that  this  family  belongs  to  B.  bp.  heteris. 

Whether  a  culture  (0645)  produced  from  seeds  sent  by  Dr.  D.  T.  Mac- 

Dougal,  from  Tucson,  Arizona,  is  likewise  identical  with  the  B.  bp.  heteris 

grown  from  seeds  collected  in  Illinois,  Ohio,  and  Long  Island,  has  not 

been  sufficiently  tested.     While  the  Tucson  plants  had  in  a  most  strongly 

marked  way  the  primary  and  secondary  lobes 

described  above,  there  were  striking'  differences 

in  the  texture  and  color  of  the  leaves,  the  form 

of  the  lobes,  and  the  form,  size,  and  texture  of 

the  stem-leaves  or  bracts  (fig'.  8).     As  only  one 

generation  of  plants  has  been  grown  from  these 

seeds,  no  proper  grounds  exist  for  attempting 

to  decide  as  to  the  permanence  of  the  differences 

exhibited,  but  it  might  be  expected  that  plants 

from  the  hot,  dry,  intensely  lighted  plains  of 

Arizona  would  display  considerable  changes  of 

a  purely  transitory  nature  on  being  transferred 

to  the  moist  atmosphere  and  relatively  dim  light 

^  of  a  more  northern  propagating-house . 

Fig.  1.— Bursa  bursa -pastorts        _.  .  ^     ,  .         .     t  ,  ^  . 

heteris.    Climax  leaves  of  a       rield  observation  indicates  that  the  type  or 

specimen  growing  near  Cold  rosette  possessed  by  Bursa  at  Tucson,  of  which 
Spring  Harbor,  Long  Island.     ,  .         ^  .  .  ^      .      . 

this  culture  was  a  fair  example,  is  the  common 

type  if  not  the  only  type  of  rosette  displayed  by  Bursa  in  that  locality  and 
westward.  Many  facts  now  at  hand  sugg^est  that  the  heteris  type  is  the  prim- 
itive type  of  rosette  from  which  the  forms  to  be  described  below  have  been 
derived,  and  the  relatively  wide  geographical  distribution  of  this  type  is 
in  strong  support  of  this  view. 

Bursa  bursa-pastoris  tenuis  n.  sp.  element. 

This  differs  from  the  preceding  type  in  several  important  features.  The 
sinuses  are  relatively  shallow,  rarely  extending  nearh-  to  the  midrib  in 
very  strongly  developed  individuals.  The  terminal  lobe  is  not  separated 
from  the  nearest  lateral  lobes  by  deep,  clean-cut  sinuses,  but  these  more 
distal  sinuses  are  relatively  shallow,  so  that  one  can  with  but  scant  pro- 
priety speak  of  the  terminal  lobe  as  a  definite  morphologfical  structure; 


BIOTYPES  AND  HYBRIDS. 


19 


there  is  no  incision  on  either  margin  of  the  lateral  lobes  and  hence  no 
rounded  lobe  corresponding  with  what  I  have  called  the  "secondary  lobe  " 
in  B.  bp.  heteris,  though  there  may  be  a  slight  expansion  of  leaf-tissue  in 
that  reg-ion,  especially  in  strongly  developed  specimens,  which  no  doubt 
corresponds  to  the  secondary  lobe  (fig.  9).  All  the  lateral  lobes  tend  to 
be  more  or  less  slender,  elongrated,  and  acute.  In  the  more  robust  speci- 
mens there  is  apt  to  be  some  secondary  lobation,  but  in  all  cases  these 
secondary  lobes  are  also  attenuate.     A  secondary  spur  directed  outward 


Fig.  S. — Bursa  biirsa-pastoris  heteris  (?)  grown    from    seed    received  from 

Tucson,  Arizona. 

and  proximad  from  near  the  base  of  the  lateral  lobes  is  often  noted  in 
well-developed  plants  belonging'  to  this  biotype  (fig.  10).  In  many  in- 
stances the  long,  slender  lobes  are  somewhat  recurved  at  the  tips,  but  in 
other  cases  all  lobes  are  practically  straight.  Under  this  type  may  belong- 
Almquist's  C.  dp.  pedemontana  (his  figf.  17),  leontodon  (fig\  3l),  dentata 
(fig.  35),  lacerata  (fig".  37),  bergiana  (fig-.  38),  laxa  (fig*.  39),  querceti  (fig-. 
40),  ramselensis  (fig-.  41),  and  linearis  (fig-.  58). 


20 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI 

Fig.  11.  Fig.  9. 


Fig.  10. 

Fig.  9. — Bursa  bursa-pastoiis  tenuis  from  my  first  pure  culture  of  this  biotype. 

Fig.  10. — Bursa  bursa-pastoris  tenuis  from  ;i  hybrid  progeny. 

Fig.  11. — Bursa  bursa-pastoris  tenuis.    A  stunted  sib  of  plant  shown  in  fig.  9. 


BIOTYPES  AND  HYBRIDS. 

Fig.  13. 


21 


Fig.  12. 
Fig.  12. — Bursa  bursa-pastoris  rhoniboidea.   Grown  in  best  light-relations  pos. 

sible  in  sky-lighted  room.     Characteristic  incisions  not  well  developed. 
Fig.  13. — Bursa  Bursa-pastoris  rJiomboidea  modified  by  growing  in  dim  light. 


22  BURSA  BURSA-PASTORTS  AND  BURSA  HEEGERI  : 

The  two  specimens  (040.19  and  040.26)  which  form  the  basis  of  all  my 
pure-bred  cultures  of  this  biotype,  gferminated  in  vessels  of  earth  over 
which  were  unpacked  Bursa  seed  which  had  been  sent  by  Prof.  H.  F.  Rob- 
erts, of  Manhattan,  Kansas,  and  as  the  soil  of  one  of  these  vessels  had  been 
sterilized  it  seems  almost  certain  that  these  plants  came  from  seed  sent 
from  Kansas.  Seeds  of  these  two  plants  were  sown  September  22,  1905, 
and  January  31,  1906,  and  tog-ether  gave  a  progreny  of  361,  all  but  4  of 
which  were  referable  without  question  to  the  type  of  the  parents.  These 
4  specimens  in  family  0519  which  were  not  quite  in  agreement  with  the  rest 
had  the  lobes  comparatively  broad  and  short,  though  still  strongly  acute, 
and  the  margins  were  slightly  crenulate  or  denticulate  (fig-.  11).  As  these 
specimens  were  somewhat  stunted,  it  is  thought  that  their  differences  may 
have  been  due  to  the  causes  producing-  the  stunting,  but  this  point  has  not 
yet  been  tested. 

Besides  these  two  pure-bred  families  of  B.  bp.  tenuis  coming  presumably 
from  Manhattan,  Kansas,  this  fonn  appeared  as  a  component  of  a  hybrid 
family  received  from  Edgewood,  New  Carlisle,  Ohio,  and  also  as  a  mem- 
ber of  a  hybrid  family  from  Chicag-o,  Illinois.  These  hybrid  families  wull 
be  discussed  later. 

Bursa  bursa-pastoris  rhomboidea  n.  sp.  element. 

This,  like  B.  bp.  heteris,  has  the  leaves  divided  to  the  midrib  and  pos- 
sesses a  similar,  more  or  less  rhombic  terminal  lobe,  set  off  by  deep  sinuses 
from  the  nearest  lateral  lobes.  Each  lateral  lobe  of  the  climax-leaves 
usually  shows  a  prominent  incision  on  its  distal  marg-in,  by  which  a  lobe 
is  formed  next  to  the  rachis,  corresponding  to  the  secondary  lobe  of  B.  bp. 
heteris,  and  in  well-developed  specimens  there  are  usually  one  or  two  sim- 
ilar incisions  on  the  proximal  marg-in.  All  lobes  formed  by  these  incisions 
are  usually  obtuse  or  broadly  ang-ular.  The  terminal  portion  of  the  lateral 
lobes  has  in  the  best-marked  examples  a  nearly  rhombic  form,  which  sug-- 
gested  the  name  (see  figs.  18  and  20,  and  plates  2  and  4).  When  grown 
under  unfavorable  conditions  the  characteristic  incisions  may  be  lost,  the 
prominent  incisions  setting-  off  the  rounded  secondary  lobe  being-  the  most 
persistent.  Almquist's  C.  bp.  siibalpina  (his  fig.  46),  densa  (fig.  51),  polyedra 
(fig-.  52),  and  perhaps  two  or  three  others  may  belong-  here. 

Three  pure  cultures  demonstrated  to  belong  to  B.  bp.  rhomboidea  have 
been  reared  from  specimens  or  seeds  brought  in  from  nature,  and  the  same 
elementary  species  has  been  included  as  hybrids  in  two  other  orig-inal 
families.  Complete  proof  of  its  elementary  character  will  appear  below  in 
the  section  on  hybrids. 

Cultures  derived  from  two  plants  g-rowing-  side  by  side  near  the  Brooklyn 
Institute's  Marine  Biologfical  Laboratory  at  Cold  Spring-  Harbor,  Long 
Island,  were  thought  for  a  time  to  belong  to  a  biotype  distinct  from  the 


BIOTYPES  AND  HYBRIDS. 


23 


more  characteristic  specimens  of  B.  bp.  rJwniboidea  secured  in  hybrid  fam- 
ihes.  These  two  individuals  were  growing'  in  a  situation  which  received 
only  the  morning"  sun.  The  leaves  were  divided  to  the  midrib,  but  the 
oblongf,  obtuse  lateral  lobes  frequently  had  no  incisions,  thoug'h  in  some 
cases  the  middle  lobes  of  the  leaves  had  the  more  characteristic  incision  on 
the  distal  margin  and  less  frequently  a  similar  incision  occurred  on  the 
proximal  margin.  The  lobes  were  rather  distant  from  each  other,  gfivingf 
a  very  unique  appearance  to  these  plants.  Seeds  of  these  two  supposed 
sibs  were  sown  on  June  6  and  14,  1905.  The  first  of  these  (040.1)  pro- 
duced 65  offspring-,  which  were  studied  in  their  relation  to  different  condi- 
tions of  environment  in  order 
to  gfet  a  clue  to  the  suscepti- 
bility of  Bursa  to  immediate 
modification  by  variations  in 
the  chemical  and  physical  con- 
ditions of  the  soil,  differences 
in  soil-moisture,  atmospheric 
humidity,  intensity  of  light, 
etc. 

Specimens  which  were  grown 
under  as  favorable  conditions 
as  the  sky-lig-hted  room  pro- 
vided were  essentially  identi- 
cal with  the  parent  (fig.  12). 
Aside  from  the  complete  sup- 
pression of  lobes  and  long 
dela}'  of  the  flowering-  period 
in  dim  lioht  (fif>"S.  13  and  14)  ^^G-  1-i- — Bursa  bursa-pastoris  rhomboidea.  A  sib 
the  most'marked  effect  of  en-        ^^>'^"ts  shown  in  figs.  1l>  and  i:;,  and  of  the  same 

age,  showing  the  complete  suppression  of  lobes, 
vironment  was  noted  m  plants       ^^^^  p,^„j  ^^^^  ^^^^^^  ^^^  ^^,^  ,^^^^^1^^  i^  ^  p^^^j^ 

kept  in  a  nearly  saturated  illuminated  corner  of  the  sky-lighted  room, 
atmosphere  attained  by  cov- 
ering with  g-lass  jars.  The  leaves  became  long,  of  very  thin  membranous 
texture,  crinkled  and  otherwise  distorted,  and  with  very  shallow  sinuses 
(fig.  15).  Plants  which  had  been  grown  in  the  dim  light  of  the  sky-lighted 
room  of  the  laboratory  and  which  showed  a  consequent  reduction  of  the 
lobes  returned  to  the  fulh-  lobed  condition  upon  being  removed  to  the  better- 
lighted  propagating-house.  Seeds  of  one  of  these  (051.19)  were  sown 
March  6,  1906,  and  produced  a  large  progeny,  only  100  of  which  were  potted 
up  and  studied.  These  were  uniform  throughout,  but  as  they  were  stunted 
by  unfavorable  conditions  in  the  propagating-house  it  was  impossible  to 
determine  with  certainty  their  relation  to  other  cultures  which  had  been 
grown  under  better  conditions. 


24 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 


The  second  of  two  original  plants  of  B.  bp.  rJiornboidea  (040.8)  produced 
330  offspring-,  all  but  5  of  which  possessed  characters  recog-nized  as  prop- 
erly belonging:  to  the  parental  type,  namely,  distinct  obtuse  lateral  lobes, 
usually  with  an  incision  on  the  distal  marg-in  and  sometimes  with  a  similar 
incision  on  the  proximal  marg-in  (fig.  16).  The  5  individuals  which  were 
aberrant  from  the  type  of  the  other  325  had  the  essential  characters  of  B. 
bp.  heteris.  As  will  be  seen  later,  heteris  is  dominant  over  rhomboidea,  and 
the  simplest  explanation  of  the  presence  of  these  5  specimens  of  heteris  in 


FiG.  ]5. — Bursa  bitrsa-pastoris  7-homboidea.     Grown  in  nearly  saturated  air 
and  moderate  illumination.     Leaves  very  thin  and  translucent. 


a  family  of  rJiomboidea  is  the  supposition  that  they  represent  chance  crosses 
between  these  two  biotypes. 

Another  culture  which  was  not  at  first  thought  to  be  referable  to  B.  bp. 
rhomboidea  was  raised  from  seeds  of  a  plant  (050.82)  growing  in  a  door- 
yard  near  Cold  Spring  Harbor,  Long  Island.  The  parent  had  rather  small 
leaves  with  crowded  lobes  and  coriaceous  texture.  The  lobes  had  a  strongly 
marked  distal  incision  and  rarely  a  slight  proximal  incision  (fig.  17).  This 
plant  was  removed  to  the  greenhouse  April  18,  1906,  and  the  flowers  were 


BIOTYPES  AND  HYBRIDS. 


25 


g-iiarded  ag'ainst  cross-pollination.  The  150  offspring:  (06190)  were  some- 
what diseased  and  stunted  for  a  time,  but  subsequently  developed  un- 
doubted characters  of  B.  bp.  rhomboidea  (fig".  18). 

Bursa  bursa-pastoris  simplex  n.  sp.  element. 

This  biotype  is  like  /?.  bp.  ton/is  in  that  the  sinuses  never  reach  the  mid- 
rib, but  it  differs  in  having-  mostly  simple  rounded  or  triangfular  acutish 
lobes,  not  attenuate.  No  incisions  are  seen  in  the  lobes  and  there  is  no 
secondary  lobing",  even  in  the  most  vig^orous  specimens,  except  some  slig-ht 


Fig.  10,.— Bursa  bursa-pastoris  rhomboidea  from  the  second  original  family. 

denticulation  (fig-.  19).  I  am  in  doubt  as  to  whether  any  of  Almquist's 
named  forms  can  be  referred  to  this  biotype,  thoug-h  it  is  possible  that  his 
C.  bp.  haiiniensis  (his  fig's.  33  and  34)  is  identical  with  my  B.  bp.  simplex. 
His  C.  bp.  gallica  (fig".  62)  and  several  others  in  whicli  lobes  are  almost  or 
quite  absent  might  be  placed  here,  but  in  most  cases  his  fig'ures  show  evi- 
dence that  the  plants  had  lost  their  characteristic  lobing:  through  some  un- 
favorable cultural  conditions,  and  if  this  was  the  cause  of  the  reduction  of 
the  lobes,  such  specimens  might  belong-  to  any  other  biotype. 


26 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI: 


Two  original  cultures  of  B.  bp.  simplex  were  grown  from  seeds  col- 
lected at  Edg-ewood,  New  Carlisle,  Ohio,  by  J.  Marion  Shull,  June  2  to  17, 

1905.  In  the  parents  of  both  these  cultures  (040.5  and  040.6)  the  lobes 
were  undivided  and  not  elong-ated,  thougfh  they  were  somewhat  triang-ular- 
acutish.  One  of  these  (040.6)  had  a  more  tapering-  apex  than  the  other 
and  its  lateral  lobes  were  more  divaricate  and  more  acute,  but  their  progf- 
enies  were  g-enerally  indistingaiishable  from  each  other.  Only  one  speci- 
men among- the  offspring-  of  040.6  had  the  long-,  tapering-  apex  of  the  parent, 
this  fact  apparently  showing-  the  difference  between  the  two  parents  in  this 
regfard  to  be  a  simple  fluctuation. 

These  two  lots  of  seed  were  sown  June  26,  1905.  The  first  (040.5) 
produced  170  plants  which,  except  for  some  slight  fluctuations,  were  evi- 
dently of  a  single  type.  A  second  generation  of  44  plants  raised  from  the 
seeds  of  an  average  specimen  (055.24)  kept  strictly  to  the  same  type. 
The  other  original  culture  (056)  consisted  of  425  specimens,  all  but  5  of 

which  were  typical  B.  bp.  simplex.  The  5 
aberrant  specimens  belonged  to  B.  bp.  rhom- 
boidea  (fig.  20),  whose  presence  in  this  family 
was  assumed  to  be  due  to  chance  crosses  in 
nature.  This  assumption  was  tested  by  rear- 
ing a  family  from  seeds  of  one  of  these 
(056.130).  Its  hybrid  character  was  fully 
demonstrated,  and  the  results  are  given  in 
Fig.  \1.- Bursa  bursa-pastoris  detail  in  the  section  devoted  to  hvbrids  (p.  42). 
rhomboidea.    Climax-leaves   of        _  ^  .  ,  "  ^  , 

a  plant  growing  in  a  dooryard  ^ome  fluctuation  was  observed  among  the 
near  Cold  Spring  Harbor,  Long  specimens  of  B.  bp.  simplex.  A  very  few 
Island.  somewhat  stunted  specimens  had  the  leaves 

smaller  than  normal,  somewhat  shining,  and  with  the  lobes  more  crowded. 
One  of  these  (055.103)  was  tested.     Fully  guarded  seeds  were  sown  July  18, 

1906,  and  produced  48  specimens,  all  but  one  of  which  were  typical  B.  bp.  sim- 
plex, the  one  slightly  aberrant  specimen  having  a  more  coriaceous  texture 
and  slightly  more  distant  lobes  than  the  others.  It  is  probable  that  this 
also  represents  a  mere  fluctuation.  Other  variations  among  the  members 
of  the  original  families  have  not  been  fully  tested  as  yet.  Thus  in  one  the 
leaves  were  broader  than  in  the  usual  form,  in  another  the  sinuses  were 
deep  and  the  lobes  rather  long,  strongly  divaricate,  and  acutish.  There 
can  be  little  doubt  that  these  are  fluctuations  which  would  be  but  slightly 
if  at  all  apparent  in  their  offspring. 


050.82 


BIOTYPES  AND  HYBRIDS. 


A  VARIABLE  SERIES  OF  DOUBTFUL  SIGNIFICANCE. 


27 


Besides  the  four  forms  named  and  described  above,  which  have  been 
shown  to  breed  true  to  type  with  only  sHg-ht  fluctuations,  most  of  my  at- 
tention has  been  given  to  a  series  of  cultures  whose  behavior  has  been  up 
to  the  present  time  quite  baffling".  Some  time  I  hope  to  understand  this 
g-roup  better,  and  I  shall  then  have  more  to  say  about  it,  but  its  behavior  is  in 
such  striking  contrast  to  that  of  the  biotypes  already  described  that  it 
seems  only  fair  to  give  a  short  epitome  of  my  results  as  they  now  appear. 


Fig.  18. — Bursa  bur sa-pastorisrhomboidea  grown  from  guarded  seed 
of  the  plant  shown  in  fig.  17. 

Two  specimens  (040.2  and  040.7)  were  taken  into  the  sky-lighted  room 
from  different  habitats  near  Cold  Spring  Harbor,  Long  Island,  in  the  very 
beginning  of  these  cultures,  April  15  to  20,  1905,  and  allowed  to  ripen  seed. 
The  aspect  of  these  two  specimens  was  very  diverse.  One  (040.2)  was 
robust  and  had  rather  firm,  thickish  leaves  with  4  or  5  pairs  of  oblong,  ob- 
tuse, wavy  lobes,  while  the  other  (040.7)  had  a  small  rosette  with  thin, 
flat  leaves  and  few  triangular  lobes.  Notwithstanding  these  differences, 
the  offspring  of  the  two  plants,  as  followed  in  numerous  cultures  through 
several  successive  g-enerations,  were  indistingtiishable  from  each  other. 


28 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 


The  seed  of  the  first  of  these  (040.2)  was  sown  June  6,  1905,  and  gave 
a  prog'eny  of  262  individuals  not  quite  equal  to  each  other  and  not  then  well 
understood,  because  these  were  my  first  cultures  of  Bursa.  These  plants 
were  nearly  uniform  in  their  membranous  texture,  smooth  surface,  and 
rather  light-g'reen  color,  but  in  the  character  of  the  lobation  considerable 
diiTerences  were  noted.  The  most  common  form,  and  that  considered  in 
consequence  to  be  typical,  had  only  obtuse  lobes  resembling;  B.  bp.  simplex. 
Besides  these  obtuse-lobed  specimens  there  were  about  4  which  had  the 
lobes  decidedly  acute  or  elong'ated. 


Fig.  10. — Bursa  bursa-pastoris  simplex  grown  from  seeds  received  from 
Edgewood,  New  Carlisle,  Ohio. 

Seeds  of  the  other  original  specimen  (040.7)  were  sown  May  8,  1905,  and 
produced  70  offspring,  about  40  of  which  were  obtuse-lobed.  The  rest 
varied  through  acute  lobes  not  elongated,  somewhat  elongated  acute  lobes 
with  slight  serration  on  both  margins  to  forms  with  well-marked  attenua- 
tion of  the  lobes  similar  to  those  of  B.  bp.  tenuis  described  above.  Most  of 
my  cultures  of  Bursa  have  been  made  to  determine  the  status  of  the  varia- 
tions in  the  offspring  of  these  two  plants  and  their  succeeding  generations. 

Before  taking  up  the  discussion  of  particular  cultures,  it  may  be  stated 
that  these  variations  from  obtuse  lobes  and  shallow  sinuses  to  more  or  less 


BIOTYPES  AND  HYBRIDS. 


29 


attenuate  lobes,  and  many  other  variations,  have  behaved  in  many  respects 
as  if  they  were  the  normal  fluctuations  of  a  single  biotype.  While  in  cer- 
tain cases  there  appeared  to  be  a  marked  capacity  of  one  or  the  other  ex- 
treme to  transmit  its  character  to  its  offspring-,  the  usual  result  of  breeding- 
any  one  of  these  variations  was  a  prog-eny  g-iving-  ag-ain  the  whole  rang-e  of 
fluctuation,  or  a  considerable  portion  of  it.  Time  and  ag:ain  forms  were 
picked  out  so  different  from  their  sibs  that  they  were  thoug'ht  to  represent 
distinct  elementary  species,  but  breeding-tests  showed  that  their  offspring- 
return  completely  to  the  usual  condition  of  the  other  related  families. 


Fig.  20. — Btirsa  bursa-pastoris  rhomboidea  from  a  family  of  B.  bp.  simplex. 
A  half  sib  of  the  plant  shown  in  fig.  li). 


Over  100  pedig-reed  families,  including"  more  than  15,500  individuals 
derived  from  the  two  plants  (040.2  and  040.7),  have  now  been  studied,  and 
of  these  families  considerably  more  than  half  ranged  between  forms  with 
very  obtuse  lobes  and  others  having-  greater  or  less  attenuation  of  the  lobes, 
the  extreme  developments  in  the  latter  direction  being-  scarcely  distinguish- 
able from  pure-bred  B.  bp.  tenuis;  17  of  the  remaining  families,  in  which  no 
attenuate-lobed  element  was  noted,  had  been  injured  by  too  long  crowding 


30  BURSA  BURSA-PASTORIS  AND  BURSA-HEEGERI  : 

in  the  seed-pans.  This  cause,  with  the  ravages  of  thrips,  in  many  cases 
so  affected  the  development  of  the  plants  during-  several  months  of  the 
summer  of  1906  that  it  was  often  very  doubtful  just  how  much  depend- 
ence was  to  be  placed  upon  the  observed  results.  However,  enough  of 
the  families  developed  healthily  to  render  it  not  improbable  that  all  degrees 
of  variation  between  completely  unlobed  individuals  and  those  with  highly 
developed  attenuate  lobes  may  be  normal  fluctuations  of  a  single  unit-form. 

Of  the  healthy  cultures  several  may  be  taken  as  examples  of  the  pecu- 
liar behavior  of  these  plants .  vSeveral  noteworthy  instances  were  presented 
in  which  the  conditions  of  the  parent  were  transmitted  to  the  offspring 
with  only  a  narrow  range  of  fluctuation.  Thus,  in  the  second  of  the  two 
original  families  from  which  all  of  these  variable  cultures  sprang,  there 
were  among  70  specimens  13  which  had  attenuate  lobes.  Seeds  of  1  of 
these  attenuate-lobed  specimens  (057.20)  were  sown  October  9,  1905,  and 
produced  681  plants,  all  but  1  of  which  showed  almost  uniformly  strong 
development  of  attenuate  lobes.  One  specimen  was  entirely  free  from 
lobation  of  any  kind,  but  it  produced  no  good  seeds  and  therefore  it  was 
impossible  to  test  the  significance  of  this  unlobed  condition.  One  of  the 
attenuate-lobed  plants  of  this  family  (0520.196)  produced  35  more  or  less 
attenuate-lobed  offspring,  and  some  which  were  unlobed,  but  the  latter 
were  so  crowded  in  the  seed-pan  that  stunting  might  be  considered  the 
cause  of  the  suppression  of  attenuate  lobes.  Progenies  of  4  of  these 
stunted  individuals  (05196.134,  05196.136,  05196.137,  and  05196.147)  have 
since  been  examined.  Two  of  these  4  families  (06136  and  06137)  had 
the  wide  range  of  variation  usuallj^  found  in  the  related  families,  and  the 
other  2  (06134  and  06147)  contained  unlobed  or  slightly  obtuse-lobed 
specimens.  Since  these  last  2  families  were  badly  crowded,  little  de- 
pendence is  to  be  placed  upon  this  behavior. 

Leaving  out  of  account  all  the  families  which  may  have  been  injured  by 
crowding  or  otherwise,  in  3  other  instances  the  offspring  were  uniform 
and  in  fair  agreement  with  the  parent.  All  3  of  these  belonged  to  the 
first  of  the  2  original  families.  Seeds  of  a  well-developed  obtuse-lobed 
individual  (052.24)  sown  Januarj^  16,  1906,  produced  100  specimens 
of  uniform  aspect,  with  only  well-developed  obtuse  lobes.  An  obtuse- 
lobed  sib  of  the  last  (052.179)  was  the  parent  of  130  plants  of  the  same 
uniform  character,  having  well-marked  sinuses,  but  acutish,  not  elongated 
lobes.  One  of  these  (05179.170)  produced  about  30  obtuse-lobed  offspring, 
but  thrips  injured  them  so  much  that  their  characterization  was  unsafe. 
Another  obtuse-lobed  plant  (052.193),  a  sib  of  052.24  and  052.179  just 
described,  gave  a  progeny  of  125  plants  of  veryimiform  aspect  throughout 
and  always  with  well-developed  sinuses  and  obtuse  lobes. 

In  all  these  exceptional  cases  there  seems  to  be  a  consistent  behavior 
in  that  the  parental  character  dominates  the  entire  progeny,  but  in  each 


BIOTYPES  AND  HYBRIDS.  31 

such  case  there  were  sibs  having-  the  same  external  characteristics,  which 
behaved  in  an  altog^ether  different  way  when  bred.  Thus,  a  sib  (057.25) 
of  the  attenuate-lobed  specimen  (057.20)  noted  above  as  producing-  off- 
spring- uniformly  of  the  parental  type,  was,  like  that  plant,  attenuate-lobed, 
but  its  offspring-,  instead  of  agreeing  with  the  parent,  consisted  of  1  plant 
entirely  without  lobation,  168  with  obtuse  lobes,  186  with  some  of  the  lobes 
slightly  elong-ated,  and  415  with  attenuate  lobes.  This  result  looks  very 
much  like  a  case  of  Mendelian  inheritance  if  we  assume  that  the  parent 
was  a  DR,  but  it  is  not  at  all  in  accord  with  such  an  assumption  that  in 
like  inanner  obtuse-lobed  sibs  of  the  plants  whose  entire  progenies  were 
characteristically  obtuse-lobed  as  described  above  have  produced  offspring- 
ranging-  from  the  totally  unlobed  condition  to  the  well-marked  attenuate- 
lobed  extreme.  For  example,  one  of  these  obtuse-lobed  plants  (052.182) 
whose  seeds  were  sown  February  12,  1906,  produced  a  family  of  471,  of 
which  337  were  observed  to  have  the  following  composition  :  11  were  wholly 
unlobed,  43  were  unlobed  in  the  distal  half  of  the  leaf,  but  had  small  trian- 
gular lobes  in  the  proximal  half,  89  were  obtuse-lobed  throughout,  102  had 
some  lobes  slig-htly  elong-ated,  and  192  had  some  lobes  strongly  attenuate. 

Besides  a  few  families  that  were  left  crowded  too  long-  in  the  seed-pans 
to  allow  of  a  satisfactory  estimation  of  the  foliar  characteristics,  15  addi- 
tional families  were  reared  from  parents  which  had  attenuate  lobes  and  all 
g-ave  uniformly  the  same  result,  namely,  progenies  showing  the  complete 
rang-e  of  variation  from  a  wholly  unlobed  condition  to  the  attenuate  condi- 
tion of  the  parent;  17  families  from  obtuse-lobed  parents  had  the  same 
composition,  as  did  also  7  families  from  unlobed  parents. 

Besides  these  variations,  which  can  be  easily  arranged  in  a  simple  linear 
series,  there  were  noted  several  variations  of  so  definite  a  character  as  to 
lead  to  the  attempt  to  seg-reg-ate  them  as  distinct  forms.  Thus  one  form 
which  appeared  in  several  families  {e.  g.,  0527,  05182,  etc.)  was  charac- 
terized by  a  distal  unlobed  half  of  the  leaf  and  a  proximal  half  with  tri- 
angular lobes.  Another  g-roup  of  specimens  (0623)  of  strikingly  uniform 
appearance  had  very  robust  rosettes  with  leaves  broad,  obtuse,  and  entirely 
unlobed  except  for  the  presence,  occasionally,  of  a  few  shallow  triangailar 
lobes  at  the  very  base. 

If  further  breeding  should  confirm  the  conclusion  that  these  families 
belong  to  a  single  biotype,  including  within  its  normal  rang-e  of  fluctuation 
individuals  with  obtuse  lobes  or  no  lobes  at  all  and  others  with  strongly 
marked  attenuate  lobes,  this  form  would  be  indistingaiishable  in  some  of 
its  phases  from  B.  bp.  simplex,  and  in  other  phases  it  would  closely  re- 
semble B.  bp.  tenuis.  Only  breeding-tests  could  safely  distinguish  these 
several  types,  and  so  long-  as  the  practical  taxonomist's  work  consists  en- 
tirely in  the  classification  of  individuals  as  they  occur  in  nature,  he  would 
be  justified  no  doubt  in  refusing-  to  recognize  such  elementary  species  as 


32  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

of  working  value,  but  in  the  taxonomy  of  the  future  they  must  be  reckoned 
with,  because  they  are  the  real  natural  entities  with  which  all  students  of 
biology  must  deal.  The  old  name  and  the  old  delimitation  of  Bursa  biirsa- 
pastoris  may  remain  as  the  only  thing  practicable  for  the  amateur  collector  of 
plants,  but  the  morphologist,  the  physiologist,  the  ecologist,  and  the  evolu- 
tionist must  be  more  discriminating.  It  appears  to  me  that  systematic 
botany  stands  at  the  parting  of  the  ways.  Either  it  is  to  be  left  stranded 
as  a  caterer  to  the  amateur  or  it  must  adopt  the  cultural  method  in  lieu  of 
the  herbarium  method,  which  has  until  recently  held  almost  supreme  sway 
among  systematists. 

I  am  not  fully  convinced,  however,  that  the  variable  families  here  de- 
scribed belong  to  a  single  biotype.  The  fact  that  both  extremes  have  in 
certain  cases  bred  true  leads  to  the  question  whether  there  is  not  some  way 
of  accounting  for  the  anomalous  behavior  of  the  other  families  on  the 
ground  of  hybridization.  I  believe  that  such  an  explanation  can  be  foiind 
on  the  principle  of  latent  characters  which  I  have  recently  discussed  else- 
where (Shull,  1908).  At  certain  times  the  cultures  have  become  very 
unhealthy  on  account  of  the  attacks  of  thrips  and  other  causes,  such  as 
overheating  during  the  summer,  crowding  in  the  seed-pans,  etc.,  and  the 
specimens  weakened  in  this  way  may  very  well  have  failed  to  show  their 
distinctive  characters,  owing  to  what  I  have  called  "latency  due  to  fluc- 
tuation," thus  destroying  completely  the  ratios  by  which  hybridization 
phenomena  would  have  been  rendered  evident. 

Whether  these  plants  belong  to  a  single  biotype  of  wide  variability  or 
to  a  hybrid  group  whose  nature  has  been  obscured  by  latency  will  have  to 
await  further  cultures.  Lotsy  (1906)  also  found  that  while  some  forms 
bred  true  with  very  slight  fluctuation,  the  variability  of  others  was  consid- 
erable. If  hybridization  with  latency  accounts  for  the  behavior  of  my 
variable  families  it  may  also  perhaps  account  for  a  similar  condition  in 
Lotsy 's  material.  Whatever  the  situation  may  be  in  these  highly  variable 
cultures,  it  is  perfectly  certain  that  the  true-breeding  forms  are  distinct 
and  elementary,  as  will  be  more  fully  demonstrated  in  the  next  section. 

HYBRIDS  BETWEEN  BIOTYPES  OF  BURSA  BURSA-PASTORIS. 

Although  artificially  produced  hybrids  between  the  several  biotypes  above 
described  have  been  studied  as  yet  only  in  the  first  generation,  except  in 
the  case  of  one  F2  family  to  be  described  hereafter  (see  page  42),  I  have 
had  under  observation  31  hybrid  families  belonging  to  a  second  or  later 
generations.  My  good  fortune  in  being  able  to  report  on  these  second- 
generation  hybrids  is  due  to  the  facts  that  two  of  the  original  families  were 
the  offspring  of  hybrid  individuals  growing  in  nature  and  several  other 
individuals  in  the  original  cultures  were  obviously  produced  by  pollination 
from  another  biotype.     The  first  of  these  natural  hybrids  (040.4)  was  col- 


BIOTYPES  AND  HYBRIDS.  33 

lected  by  J.  Marion  Shull  at  Edg-ewood,  New  Carlisle,  Ohio,  May  26-28, 
1905.  This  specimen  was  robust  and  the  lobes  of  the  leaf  were  lanceolate, 
with  prominent  dentations  on  both  proximal  and  distal  margins. 

The  seeds  were  sown  June  26,  1905,  and  produced  a  progeny  of  284 
individuals,  among'  which  there  was  such  a  variety  of  form  and  aspect  as 
to  make  them  at  that  time  entirely  inexplicable,  because  none  of  the  unit- 
differences  between  the  forms  was  then  known  and  there  was  no  possibility 
of  discriminating  between  minor  fluctuations  and  the  distinguishing-  char- 
acteristics of  the  several  elementary  species.  Efforts  were  made  to  arrange 
these  plants  into  groups  that  would  be  strictly  homogeneous,  and  in  this 
way  no  fewer  than  25  categories  were  established.  Even  these  were  not 
sufficient,  and  finally  a  considerable  number  of  individuals  that  could  not 
be  classed  with  any  of  these  were  preserved  in  order  that  they  might  be 
studied  at  any  subsequent  time  at  which  it  should  be  desired  to  work  out 
their  variations  and  their  relationship  to  each  other  and  to  the  rest  of  the 
family. 

Fortunately  the  distinguishing  marks  of  each  of  these  several  groups 
had  been  noted  with  sufficient  thoroughness  that  later,  when  the  differen- 
tiating characteristics  of  the  several  elementary  species  were  better  under- 
stood, it  was  possible  to  go  back  and  reclassify  the  several  categories  in 
such  a  manner  as  to  determine  approximately  the  proportions  in  which  the 
different  elementary  species  were  present.  The  maze  of  different  forms 
had  proved  so  baffling  at  every  attempt  at  a  satisfactory  classification  that 
the  result  of  the  redistribution  of  the  several  forms  in  the  light  of  knowl- 
edge subsequently  gained  occasioned  much  surprise.  There  were  among 
the  284  individuals  composing  the  family,  114  B.  bp.  heteris,  47  B.  dp.  tenuis, 
53  B.  bp.  rhomboidea,  16  B.  bp.  simplex,  and  54  which,  because  of  certain 
deficiencies  in  the  original  notes,  could  not  be  reclassified  with  certainty, 
but  nearly  all  of  which  were  certainly  fluctuations  of  B.  bp.  heteris. 

It  need  only  be  assumed  that  of  these  54  doubtful  individuals  46  were 
B.  bp.  heteris,  6  B.  bp.  tenuis,  and  2  B.  bp.  simplex  to  make  the  ratios  of 
these  several  elementary  forms  agree  exactly  with  a  frequently  observed 
Mendelian  ratio,  9:3:3:1.  Fortunately  most  of  these  doubtful  specimens 
were  preserved  in  the  herbarium  and  were  thus  available  for  study.  Of  51 
thus  preserved,  37  were  B.  bp.  heteris,  9  B.  bp.  tenuis,  1  B.  bp.  rhomboidea, 
and  4  of  doubtful  affinity,  these  latter  probably  belonging  to  the  heteris 
group  also,  but  representing  cases  of  incomplete  dominance. 

The  ratio  9:3:3:1  is  the  normal  one  for  the  second  generation  of 
typical  Mendelian  di-hybrids,  /.  <?.,  hybrids  between  forms  that  differ  from 
each  other  in  two  unit-characters .  All  of  these  different  elementary  species 
were  supposed  to  differ  from  each  other  by  single  units  until  this  family 
was  worked  out,  because  in  nearly  all  the  other  hybrid  families  the  simple 
ratio  of  3  to  1  appeared. 


34  BURSA  BURSA-PASTORI.S  AND  BURSA  HEEGERI  : 

As  soon  as  it  was  demonstrated  that  there  are  two  unit-differences 
involved  in  these  forms  it  was  not  difficult  to  discover  in  what  these  two 
units  consist.  One  of  them  is  the  elongation  of  the  primary  lobes,  the 
other  the  extension  of  the  sinuses  to  the  rachis  and  the  presence  of  rounded 
secondary  lobes  in  the  distal  axils  of  the  primary  lobes.  If  we  represent 
the  first  by  A  and  its  absence  by  a,  and  the  second  by  B  and  its  absence 
by  b,  then  we  may  disting-uish  the  four  members  of  the  ratio  as 
AB  :  Ab  :  aB  :  ab.  In  the  first,  AB,  is  denoted  the  presence  both  of  the 
elongated  primary  and  the  rounded  secondary  lobe,  the  distinguishing 
characteristics  of  B.  bp.  heteris ;  in  the  next,  Ab,  the  primary  lobes  are 
attenuate,  but  the  rounded  secondary  is  not  present,  this  being  the  char- 
acter of  B.  bp.  tenuis;  in  the  third,  aB,  the  rounded  secondary  is  present, 
but  the  primary  is  not  elongated,  giving  B.  bp.  rhomboidea;  while  in  the 
fourth,  ab,  both  the  secondary  and  the  elongation  of  the  primary  are  absent, 
resulting  in  B.  bp.  simplex. 

The  most  remarkable  corollary  of  this  composition  of  these  forms  is  the 
possible  resolution  and  recomposition  of  the  several  elementary  species. 
Thus,  a  cross  between  pure-bred  B.  bp.  heteris  and  pure-bred  B.  bp.  sim- 
plex, or  a  cross  between  pure-bred  B.  bp.  tenuis  and  pure-bred  B.  bp. 
rhomboidea  should  again  give  in  the  second  generation  all  four  forms  in  the 
ratio  9:3:3:1.  The  correctness  of  this  prediction  awaits  the  test  of 
further  experimentation. 

Of  the  families  representing  the  third  generation  of  this  hybrid  20  have 
now  been  studied,  and  the  results  have  agreed  well  with  the  interpretation 
here  given  of  the  constitution  of  the  original  family.  These  20  families 
are  briefly  described  below  in  the  order  of  the  pedigree-numbers  by  which 
they  were  designated  in  the  cultures  and  original  notes. 

054.184:  This  plant  was  described  as  of  the  same  general  type  as  B. 
bp.  heteris,  with  long,  slender  primary  lobes  and  a  rounded  secondary  lobe 
in  the  distal  axil  of  the  primary.  The  secondary  lobe  usually  bore  1  or  2 
sharp,  erect  lobes.  vSeeds  of  this  plant  were  sown  February  23,  1906,  and 
produced  about  200  offspring.  Owing  to  crowding  of  the  propagating-house 
at  that  season  of  the  year,  only  100  plants  were  potted  and  kept  for  obser- 
vation, the  remainder  being  discarded  with  the  seed-pan.  The  specimens 
potted  up  were  taken  from  the  pan  in  such  manner  as  to  prevent  possible 
selection  of  individuals.  Of  82  specimens  noted,  80  were  regarded  as  B. 
bp.  heteris,  and  2  as  near  B.  bp.  tenuis,  but  many  of  the  specimens  were 
stunted  and  it  appears  certain  now  that  some  specimens  of  B.  bp.  tenuis 
were  looked  upon  as  stunted  B.  bp.  heteris,  because  the  importance  of  the 
secondary  lobe  was  not  fully  appreciated  at  that  time.  That  the  parent 
plant  (054.184)  was  really  a  hybrid  between  B.  bp.  heteris  and  B.  bp.  tenuis 
is  further  indicated  by  the  behavior  of  one  of  its  offspring,  which  was 


BIOTYPES  AND  HYBRIDS.  35 

saved  as  a  seed-plant  under  the  number  05184.145,  and  whose  progenj-  is 
next  described. 

05184.145  :  This  plant  had  the  climax  leaves  of  B.  bp.  /leteris,  but  in  the 
later  rosette-leaves  the  rounded  secondary  was  lost  and  the  lobes  then  had 
one  to  several  rather  sharp  incisions  on  both  proximal  and  distal  margins. 
Seeds  of  this  plant  were  sown  Jiine  20,  1906,  and  gave  a  progeny  consist- 
ing of  158  B.  bp.  heteris  and  58  B.  bp.  tenuis,  or  2.72  :  1. 

054.185 :  This  plant  was  selected  as  nearly  the  equivalent  of  its  parent 
(040.4).  It  had  the  lobes  about  evenly  tapering  and  equally  serrated  on 
both  margins.  Its  seeds  were  sown  February  23,  1906,  and  gave  a  large 
progeny,  which  at  one  time  suffered  greatly  from  unsatisfactory  conditions 
for  growth  during  the  summer  months.  Of  these  stunted  specimens  236 
were  discarded  as  modifications  of  B.  bp.  tenuis,  but  50  equivalent  speci- 
mens transferred  to  larger  pots  showed  that  this  determination  was  wholly 
unsatisfactory.  Well-grown  specimens  belonging  to  this  family  were  later 
shown  to  consist  of  33  B.  bp.  heteris,  48  B.  bp.  tenuis,  5  B.  bp.  rhomboidea, 
and  14  B.  bp.  simplex.  It  is  plain  that  the  manipulation  of  this  family  was 
such  that  nothing  of  value  can  be  derived  from  the  ratios,  but  it  is  worth 
noting  that  this  plant,  which  was  considered  the  equivalent  of  its  parent, 
produced  a  family  composed  of  the  same  four  forms  which  had  appeared  in 
the  parental  famih^ 

054.186 :  This  plant  had  the  lobes  divided  into  rather  narrow  secondary 
lobes  by  deep  incisions  on  both  distal  and  proximal  margins.  It  had  some 
of  the  characters  of  both  B.  bp.  heteris  and  of  B.  bp.  rhotnboidea,  but  the 
resemblance  to  neither  was  very  striking.  The  seeds  were  sown  February 
23,  1906,  and  produced  a  progeny  which  was  judged  to  consist  of  30  B.  bp. 
heteris,  22  B.  bp.  rhomboidea,  and  45  intermediate  between  these  two. 
These  intermediate  specimens  differed  from  B.  bp.  heteris  in  the  prominent 
incisions  on  the  proximal  margin  of  the  primary  lobes.  As  these  interme- 
diate forms  pretty  completely  bridged  the  gap  between  the  two  parental 
biotypes,  the  wide  departure  from  the  ratio  1:2:1  was  doubtless  only  ap- 
parent, being  due  probably  to  the  uncertainty  of  the  judgment  in  separating 
the  several  classes.  This  shows  that  B.bp.  heteris  is  not  always  completely 
dominant  over  ^.  bp.  rhomboidea,  and  the  same  fact  will  be  noted  in  several 
other  cases. 

054.190:  This  was  a  well-marked  specimen  of  B.  bp.  rhomboidea.  Its 
seeds  were  sown  February  28,  1906,  and  gave  a  progeny  of  nearly  a  thou- 
sand. Owing  to  the  limitations  of  space  in  the  propagating-house,  only 
123  of  these  plants  were  potted  up  for  observation.  One  of  these  died.  Of 
the  remainder,  all  but  1  were  B.  bp.  rhomboidea:  1  differed  from  the  rest 
of  the  family  in  the  elongation  of  the  terminal  portion  of  the  lobes,  this 
being  recognized  as  the  character  of  the  heterozygous  condition  of  the 


36  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

hj^brid  between  B.  bp.  /leteris  and  B.  bp.  rhomboidea.  Had  B.  bp.  hetcris 
been  a  normal  component  of  this  family  it  would  have  made  up  a  large 
portion  of  it,  instead  of  less  than  1  per  cent.  As  none  of  the  earlier  cul- 
tures were  g^uarded  against  cross-pollination,  it  seemed  more  reasonable 
to  consider  this  one  specimen  the  result  of  a  chance  cross  produced  by 
pollen  from  B.  bp.  hetcris,  which  was  abundantly  represented  in  the  house 
at  the  same  time.  To  test  further  the  character  of  the  normal  components 
of  this  family  as  well  as  of  this  one  atypic  individual,  the  following  two 
cultures  were  made  : 

05190.167:  A  typical  B.  bp.  rhomboidea,  the  type  of  the  last  family, 
whose  seeds  were  sown  July  4,  1906,  gave  29  plants,  all  B.  bp.  rhomboidea, 
showing  the  expected  purity  of  this  extracted  form.  The  pollination  of 
this  plant  had  been  carefully  guarded. 

05190.178:  This  was  the  atypic  plant  described  under  the  last  familj^ 
but  one.  The  pollination  was  guarded,  and  the  seeds  sown  July  11,  1906, 
resulted  in  a  progeny  which  contained  29  B.  bp.  hetcris,  29  B.  bp.  rhom- 
boidea, and  37  intermediate  between  the  two.  The  deficiency  of  the  inter- 
mediate or  heterozygous  class  is  again  evidence  of  the  difficulty  of  distin- 
guishing between  the  extreme  variations  of  these  and  either  pure-bred 
parent.  The  result  here  secured  is  sufficient  demonstration  of  the  simple 
hybrid  character  assumed  for  the  parent. 

054.208  :  This  plant  had  the  attenuate  primary  lobes  rather  distant  and 
entirely  lacked  rounded  secondary  lobes.  The  sinuses  of  the  earlier  climax 
leaves  did  not  nearly  reach  the  midrib,  but  later  leaves  were  cut  more 
deeply.  Seeds  of  this  typical  B.  bp.  tenuis  were  sown  March  2,  1906,  and 
gave  a  progeny  consisting  of  24  B.  bp.  temiis  and  1  B.  bp.  heteris.  As  B. 
bp.  heteris  is  dominant  over  B.  bp.  tenuis,  it  seems  likely  that  the  specimen 
of  the  fonner  biotype  in  this  family  represents  another  instance  of  chance 
crossing,  since  the  pollination  was  unguarded,  as  in  the  other  early  cul- 
tures. If  this  assumption  is  true,  this  family  represents  another  instance 
in  which  an  extracted  form  breeds  true. 

054.209 :  In  the  climax  leaves  this  plant  had  the  typical  form  of  B.  bp. 
heteris.  Later  leaves  of  the  rosette  showed  a  nearly  or  quite  complete  siip- 
pression  of  the  rounded  secondary  lobe.  Seeds  were  sown  March  2,  1906, 
but  very  few  plants  were  secured.  These  were  not  well  studied,  but  were 
observed  to  be  heterogeneous,  though  most  were  related  to  B.  bp.  heteris, 
the  parent  form. 

054.210  :  This  plant  was  a  marked  example  of  B.  bp.  temiis,  remarkable 
for  the  extreme  attenuation  of  its  lobes.  The  seeds  were  sown  March  2, 
1906,  and  of  the  31  plants  capable  of  classification,  27  were  B.  bp.  te7iuis  of 
strongly  marked  type,  and  4  were  B.   bp.   heteris.     Unless  these  4  were 


BIOTYPES  AND  HYBRIDS.  37 

ag-ain  the  result  of  chance  crosses,  their  appearance  here  seems  at  present 
inexplicable.  A  small  number  of  specimens  in  this  family  were  too  stunted 
by  unfavorable  conditions  to  render  classification  possible,  and  some  of 
these  may  also  have  been  B.  dp.  hcteris,  but  it  is  probable  that  most  of  these 
stunted  specimens  were  B.  bp.  tenuis. 

054.3 :  The  lobes  of  this  plant  were  attenuate  and  rather  distant,  the 
terminal  lobe  spatulate  or  obovate,  obtusish,  dentate.  vSeeds  of  this  B.  bp. 
tenuis  were  sown  March  3,  1906,  and  the  18  offspring- observed  were  doubt- 
less all  B.  bp.  tenuis,  though  there  was  considerable  variation  in  the  degree 
of  lobation,  the  least-lobed  being  stunted  and  not  greatly  different  frotn 
B.  bp.  simplex. 

054.4:  This  was  a  robust  specimen  which  had  the  earlier  climax-leaves 
similar  to  B.  bp.  heteris  and  in  the  later  rosette-leaves  had  long,  attenuate, 
acute  lobes,  deeply  cut  on  both  margins.  The  seeds  of  this  plant  were 
sown  March  3,  1906,  and  none  of  the  specimens  were  potted,  but  47  were 
cut  from  the  seed-pan.  It  was  found  that  classification  of  Bursa  from  the 
seed-pan  is  very  difficult  and  uncertain — in  some  cases  quite  impossible  — 
owing  to  crowding  and  consequent  stunting  and  suppression  of  characters. 
The  specimens  of  this  family  removed  from  the  seed-pan  were  classified 
thus:  30  B.  bp.  heteris,  6  B.  bp.  tenuis,  4  B.  bp.  rhomboidea,  and  7  interme- 
diate between  B.  bp.  heteris,  and  B.  bp.  rhomboidea.  It  is  obvious  that  this 
was  a  complex  hybrid,  and  the  number  of  specimens  available  was  too  small 
for  the  satisfactory  working  out  of  ratios.  The  excessive  proportion  of 
B.  bp.  heteris  is  easily  explainable,  because  this  form  has  its  distinguishing" 
marks  less  easily  rendered  latent  by  bad  cultural  conditions. 

054.6:  This  plant  had  all  the  lobes  simple  and  mostly  obtuse.  It  should 
be  an  extracted  recessive,  and  therefore  incapable  of  producing  any  other 
than  its  own  normal  characteristics.  Seeds  were  sown  March  3,  1906. 
All  of  the  plants  produced  seemed  to  be  B.  bp.  siinplex,  though  only  1 
was  potted.  120  specimens  cut  from  the  seed-pan  agreed  with  the  char- 
acters of  the  parent. 

054.7:  The  lobes  of  this  plant  were  rather  broadly  compound,  with  all 
the  lobes  rounded,  agreeing' with  the  characters  of  ^.  bp.  rhomboidea.  The 
seeds  were  sown  March  3,  1906.  None  of  these  were  potted,  but  43  speci- 
mens cut  from  the  seed-pan  were  typical  B.  bp.  rhomboidea;  some  specimens 
more  nearly  resembled  B.  bp.  simplex,  but  opportunity  was  wanting  to 
test  this  point,  and  a  doubt  remains  whether  these  were  true  B.  bp.  sim- 
plex, or  whether  they  were  specimens  of  B.  bp.  rhomboidea  in  which  the 
characteristic  incisions  were  wanting  because  of  crowding  in  the  seed-pan. 

054.20:  The  plant  to  which  this  number  was  assigned  had  the  long, 
clean-cut,  primary  lobe  and  well-marked  rounded  secondary  characteristic 


38  BURSA  BURSA-PASTORIvS  AND  BURSA  HEEGERI  : 

of  the  best  examples  of  B.  bp.  heteris.  The  seeds  were  sown  March  6,  1906, 
and  produced  a  uniform  progeny  of  B.  bp.  heteris,  some  fluctuation  being' 
noted  in  the  denticulation  of  the  primary  lobe.  None  of  these  were  potted, 
but  103  specimens  were  cut  from  the  seed-pan,  all  being  of  the  same  type 
as  the  parent. 

054.21:  This  was  a  specimen  of  B.  bp.  rhomboidea.  The  seeds  were 
sown  March  6,  1906.  The  plants  were  allowed  to  become  too  crowded  in 
the  seed-pan,  and  did  not  reach  their  best  development.  Their  general 
aspect  was  homogeneous,  but  examination  of  the  lobes  showed  some  similar 
\.o  B.  bp.  simplex,  others  more  like  B.  bp.  rhomboidea.  The  status  of  the 
simpIex-\\k.Q  specimens  in  this  progeny  is  not  known,  but  it  seems  probable 
that  many  of  them  were  really  B.  bp.  rhomboidea,  in  which  the  characteristic 
incisions  failed  to  develop  because  of  the  crowding  of  the  plants. 

054.25:  This  plant  was  a  well-developed  specimen  of  B.  bp.  temds,  with 
occasional  secondary  spurs  on  the  proxiinal  margin  of  the  primary  lobes, 
but  with  no  trace  of  a  rounded  secondary  lobe  in  the  distal  axils.  The 
seeds  were  sown  April  18,  1906,  and  produced  136  young  plants.  When 
examined  in  August,  after  my  return  from  2  months'  absence  in  California, 
54  were  dead.  The  rest  were  all  B.  bp.  temds,  or  modifications  of  it,  except 
3  which  were  B.  bp.  heteris.  Unless  the  latter  were  the  result  of  chance 
crosses  their  origin  is  not  understood. 

054.26:  This  specimen  belonged  to  the  most  distinct  type  of  B.  bp.  heteris. 
The  seeds  were  sown  April  18,  1906,  and  produced  173  offspring.  About 
20  died  and  the  remainder  belonged  to  B.  bp.  heteris,  and  B.  bp.  tenuis  in 
the  ratio  119  :  34  or  3.5  :  1.  Some  of  the  specimens  classed  as  B.  bp.  heteris 
had  the  primary  lobe  rather  strongly  incised  on  both  margins.  This  is 
a  character  frequently  seen  in  robust  specimens  of  B.  bp.  temds,  and  it  is 
possible  that  this  represents  a  slight  lack  of  complete  dominance  of  B.  bp. 
heteris  over  B.  bp.  temds. 

054.27:  This  plant  had  large,  wide-spreading,  rather  thickish,  stiff, 
strongly  bipinnatifid  leaves,  with  all  the  lobes  tapering  and  acute,  agreeing 
thus,  in  an  essential  way,  with  the  parent  (040.4).  The  seeds  were  sown 
April  18,  1906,  and  produced  about  270  offspring.  These  are  seen  now  to 
indicate  that  the  parent  was  a  di -hybrid,  but  at  the  time  the  famih'  was  being 
studied  the  distinctness  and  the  limitations  of  the  several  forms  were  not 
sufficiently  appreciated,  and  the  notes  made  at  that  time  were  in  the  terms  of 
a  dominant  and  a  recessive  group,  the  dominant  group  having  the  sinuses 
extending  to  the  rachis  and  the  terminal  lobe  more  evenh^  rounded  and 
more  cuneate  than  in  the  recessive  group,  which  had  the  sinuses  much  less 
deep.  If  these  distinctions  were  consistentlj^  made  throughout,  the  result 
should  be  the  same  as  that  of  a  simple  Mendelian  hybrid,  since  the  former 


BIOTYPES  AND  HYBRIDS.  39 

group,  as  described,  would  include  B.  bp.  heteris  and  B.  bp.  rhomboidea,  while 
the  latter  would  contain  B.  bp.  tenuis  andiS".  bp.  sbnplex,  and  the  ratio  of  the 
two  groups  should  be  3  :  1.  The  observed  ratio  was  187  :  66  or  2.83  :  1,  a 
fair  agreeinent  with  expectation.  The  quantitative  relations  of  the  sub- 
groups can  not  be  derived  from  the  notes,  but  amongil46  of  the  dominant 
g^roup  discarded  at  one  time,  25  were  considered  B.  bp.  rhomboidea,  and  the 
most  of  the  remainder  were  intermediate  between  B.  bp.  rhomboidea  and/?. 
bp.  heteris.  According"  to  theoretical  considerations  there  should  have  been 
36  B.  bp.  rJiomboidea  in  that  number.  I  have  no  doubt  that  this  discrepancy 
was  due  to  the  fact  that  too  narrow  a  view  was  taken  of  the  fluctuations 
normal  to  the  several  recognized  forms,  so  that  some  which  were  consid- 
ered intermediate  between  B.  bp.  heteris  and  B.  bp.  rhomboidea  were  in 
reality  extracted  B.  bp.  rhomboidea.  This  is  even  more  certainly  true  with 
respect  to  B.  bp.  heteris,  for  in  the  same  group  of  146  dominants,  only  2  or 
3  were  recorded  as  B.  bp.  heteris.  All  those  having-  some  incisions  on 
the  proximal  marg-in  of  the  primary  lobe  were  considered  intermediate, 
but  pure-bred  B.  bp.  heteris  has  since  been  observed  to  possess  these  incisions 
frequently  as  a  fluctuating  character.  No  B.  bp.  simplex  was  recognized, 
as  this  elementary  species  would  have  been  thrown  with  B.  bp.  tenuis  with- 
out question  on  the  basis  of  depth  of  sinus  and  form  of  the  terminal  lobe, 
which  characters  alone  were  used  in  the  classification. 

054.28 :  This  plant  was  a  grood  specimen  of  B.  bp.  rhomboidea,  having 
the  incisions  on  both  proximal  and  distal  margins  of  the  primary  lobes, 
and  all  the  lobes  on  the  earlier  climax  leaves  rounded  or  rhomboidal. 
The  later  rosette-leaves  had  some  of  the  secondary  lobes  acutish,  but  not 
elong-ated.  The  seeds  were  sown  on  April  18,  1906,  and  325  plants  were 
potted.  Of  302  which  were  studied  later,  202  were  classified  as  B.  bp. 
rhomboidea  and  100  as  B.  bp.  simplex.  This  is  a  rather  large  departure 
from  the  expected  ratio  of  3  :  1,  but  here  again  the  excess  of  B.  bp.  simplex 
may  be  due  to  the  fact  that  the  characteristic  marks  of  B.  bp.  rhomboidea 
tend  to  disappear  under  unfavorable  conditions  of  culture,  so  that  some 
that  were  classed  as  B.  bp.  simplex  may  have  been  modified  specimens  of 
B.  bp.  rhomboidea. 

054.29:  This  plant  was  a  well-marked  specimen  of  B.  bp.  heteris, 
having-  slig-ht  denticulations  on  the  primary  lobes.  It  differed  from  the 
usual  habit  of  Bursa  bursa-pastoris  in  having  the  leafy  portion  of  the  stem 
absent.  The  seeds  were  sown  April  18,  1906,  and  produced  over  400 
young-  plants.  These  were  readily  divisible  into  two  groups — B.  bp.  heteris 
and  B.  bp.  temiis—\xi  the  ratio  319  :  95  or  3.36  :  1.  A  few  of  the  speci- 
mens of  this  family  which  were  classified  with  the  dominant  form  differed 
from  B.  bp.  heteris  in  the  lack  of  the  rounded  secondary.  They  were  like 
it,  howe-ver,  in  that  the  sinuses  extended  completely  to  the  midrib,  thus 


40  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

g-iving-  more  nearly  the  aspect  of  B.  bp.  hctcris  than  of  B.  bp.  tenuis.     I  now 
suspect  that  these  should  have  been  classed  with  B.  bp.  temds. 

In  summing-  up  these  20  hybrid  families  belonging  to  the  third  or  later 
generations,  it  appears  that  8  of  the  9  possible  combinations  are  included. 
Two  of  the  seed-plants  proved  to  be  extracted  B.  bp.  heteris,  4  were  extracted 
B.  bp.  tenuis,  2  were  extracted  B.  bp.  rhomboidea,  1  was  extracted  B.  bp. 
simplex;  3  were  again  like  the  parent  in  being  di-hybrids  which  gave  rise  to 
all  4  biotypes;  3  were  B.  bp.  heteris  X  tenuis,  2  weveB.  bp.  heteris  X  7'hom- 
boidea,  1  was  B.  bp.  r/ioniboidea  X  simplex,  and  the  characters  of  2  were  in 
doubt.  These  2  doubtful  families  (067  and  0621)  were  either  extracted  B. 
bp.  rhomboidea  or  hybrids  between  B.  bp.  r/iomboidea  and  B.  bp.  simplex, 
the  doubt  being  caused  by  the  fact  that  stunted  or  juvenile  specimens  of 
B.  bp.  rhomboidea  may  be  practically  indistinguishable  from  B.  bp.  simplex. 
In  a  few  families  the  conditions  of  the  culture  made  the  determination  of 
ratios  impracticable,  but  in  the  majority  the  approximation  to  Mendelian 
expectation  is  fairly  close.  Small  numbers  of  B.  bp.  heteris  occurred  in  3 
families  where  large  numbers  would  have  been  expected  if  that  elementary 
form  had  been  a  normal  component  of  these  hybrid  families,  since  B.  bp. 
heteris  dominates  both  B .  bp.  tenuis  and  B.  bp.  simplex,  and  to  a  slight  degree 
B.  bp.  rhomboidea  also.  The  8  specimens  of  B.  bp.  heteris  which  seemed 
out  of  place  among  more  than  2,600  individuals  included  in  these  fami- 
lies—less than  one-third  of  1  per  cent  of  the  whole — were  probably  the 
result  of  chance  crosses,  as  all  of  these  occurred  in  families  whose  polli- 
nation had  not  been  guarded. 

040.14  :  The  second  hybrid  brought  in  from  nature  was  collected  by  C. 
A.  Shull,  in  Jackson  Park,  Chicago,  in  the  summer  of  1905.  This  plant 
had  long,  acutish  lobes,  serrated  on  both  margins,  and  the  sinuses  extended 
practically  to  the  rachis.  There  was  a  very  faint  indication  of  a  rounded 
secondary  lobe  in  some  leaves.  Seeds  were  sown  December  27,  1905,  and 
produced  99  B.  bp.  heteris  and  26  B.  bp.  tenuis,  or  in  the  ratio  3.8  :  1.  The 
two  components  of  this  hybrid  were  at  first  considered  distinct  biotypes 
until  another  family  bearing  the  same  characters  as  the  dominant  form 
proved  in  the  third  controlled  generation  to  be  B.  bp.  heteris.  Subsequent 
breeding  has  demonstrated  that  these  hybrids  also  present  the  characters 
of  typical  B.  bp.  heteris  and  typical  B.  bp.  tenia's  when  grown  for  several 
generations  under  favorable  conditions.  The  peculiarities  of  the  original 
plant  and  of  the  first  g'eneration  under  culture  are  thiis  shown  to  have 
been  fluctuations  of  these  two  types.  Eight  families  of  the  third  genera- 
tion have  been  studied,  with  the  following  results  : 

0514.128:  This  was  a  specimen  of  B.  bp.  tenia's,  the  recessive  fonn, 
and  as  the  pollination  was  carefully  guarded,  it  should  have  been  expected 
to  produce  nothing  but  the  parental  form  among  the  oflfspring.     The  seeds 


BIOTYPES  AND  HYBRIDS.  41 

were  sown  June  5,  1906,  and  produced  a  uniform  proo^eny  of  145  plants, 
all  like  the  parent. 

0514.131  :  This  specimen  was,  like  the  last,  B.  hfy.  temiis.  Purely  fer- 
tihzed  seeds  were  sown  June  6,  1906,  and  the  52  plants  produced  were  all 
alike  and  like  the  parent. 

0514.146  :  This  was  of  the  dominant  type,  B.  bp.  heteris.  The  pollina- 
tion was  probably  not  gfuarded,  as  no  note  was  made  rei>-arding-  it.  The 
seeds  were  sown  June  20,  1906,  and  produced  444  oflfspring-  belong-in.o"  to 
the  dominant  and  recessive  forms  in  the  ratio  317  :  127  or  2.5  :  1.  The 
dominant  group  appeared  to  belong-  to  two  distinct  types,  with  the  pri- 
mary lobes  broader  and  less  sharp  in  the  one  than  in  the  other  in  the  ratio 
110  :  32  or  3.44  :  1.  The  sig-nificance  of  the  ratio  thus  formed  by  this 
family,  9  :  3  :  4,  is  well  known,  as  it  represents  the  simplest  modification 
of  the  ratio  for  the  second  g-eneration  of  a  di-hybrid,  but  without  further 
data  in  support  of  this  composition  for  the  family,  it  appears  advisable  not 
to  consider  its  sig-nificance  at  this  time. 

0514.161  :  This  was  a  typical  specimen  of  the  recessive  form,  B.  bp. 
tenuis,  of  which  the  pollination  was  g-uarded.  The  seeds  were  sown  July 
4,  1906,  and  produced  104  specimens,  all  of  which  were  like  the  parent. 

0514.162  :  This  was  also  an  extracted  recessive.  Guarded  seeds  were 
sown  July  4,  1906,  and  g'ave  a  prog-eny  of  15  specimens,  all  like  the  parent. 

0514.164:  A  member  of  the  dominant  gfroup,  B.  bp.  heteris.  Purely 
fertilized  seeds  sown  July  4,  1906,  produced  134  plants.  The  family  was 
badly  damagred  by  aphis,  so  that  23  were  killed  and  about  15  more  so  stunted 
as  to  make  classification  uncertain.  The  remaining-  93  belonged  to  B.  bp. 
heteris  and  B.  bp.  temiis  in  the  ratio  66  :  27  or  2.44  :  1.  In  this  family 
the  dominant  group  was  not  scrutinized  with  sufficient  care  to  determine 
whether  it  was  heterog-eneous  like  0514.146,  but  it  was  considered  of  a 
sing-le  type. 

0514.165  :  This  was  a  typical  specimen  of  the  dominant  type,  B.  bp. 
heteris.  Purely  fertilized  seeds  were  sown  Jul}'  4,  1906,  and  produced  42 
specimens  of  B.  bp.  heteris  and  13  of  B.  bp.  tenuis,  the  ratio  being  3.23  :  1. 

0514.182  :  Although  the  notes  are  defective  regarding  the  character  of 
this  plant,  the  result  of  the  breeding  test  leaves  no  doubt  that  it  was  a  re- 
cessive. Guarded  and  purely  fertilized  seeds  were  probably  sown  in  July, 
1906,  though  no  record  can  be  found  to  that  effect.  On  August  18,  1906, 
the  entire  progeny,  consisting^  of  158  specimens,  were  potted,  and  without 
exception  these  belonged  to  B.  bp.  teuids. 

These  8  families  belong-ing  to  the  third  or  later  hybrid  g-cneration  of  B. 
bp.  heteris  X  tenuis  represent  the  extracted  recessive  in  5  cases  and  the 
heterozygous  form  in  3.     The  extracted  recessives  have  bred  true  to  the 


42  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

characters  of  B.  bp.  tenuis  without  exception  in  330  individuals.  In  the 
offspring:  of  3  heterozyg-ous  plants  that  have  been  tested,  the  ratios  2.44  :  1, 
2.5  :  1,  3.23  :  1,  and  3.44  :  1  have  appeared,  and  these  are  no  doubt  as 
near  3  :  1  as  the  smallness  of  the  families  should  have  led  us  to  expect. 

056.130:  Besides  these  2  orig-inal  hybrid  families,  it  will  be  recalled  that 
certain  unexpected  individuals  oi  B.  bp.  rhoviboidea  in  one  of  the  original 
families  of  B.  bp.  simplex  (056)  were  supposed  to  be  due  to  chance  crosses 
(see  p.  26).  Seeds  of  one  of  these  (056.130)  were  sown  Jiine  6,  1906,  and 
produced  217  B.  bp.  rhomboidea  and  72  B.  bp.  simplex,  that  is,  in  the  ratio  of 
3.01  :  1,  thus  showing  the  assumption  that  they  were  of  hybrid  origin  to 
be  correct.     (See  plate  2). 

In  addition  to  the  hybrids  brought  in  from  nature  and  their  self -fertilized 
offspring-  as  represented  by  the  31  families  which  are  described  above,  7 
first-generation  hybrid  families  between  different  biotypes  of  Bursa  bursa- 
pastoris  have  been  produced  by  artificial  crossing.  Only  one  F2  family  has 
been  reared  from  these  to  the  present  time. 

0515.93  :  The  mother  of  this  cross  was  a  specimen  belonging  to  the 
second  original  family  of  B.  bp.  hcteris  (0515)  described  above  (see  p.  14). 
This  plant  was  castrated  and  pollinated  with  pollen  from  a  specimen  of  the 
first  original  culture  of  B.  bp.  tennis  (0519,  see  p.  22).  The  seeds  were 
sown  May  2,  1906,  and  produced  222  plants,  all  resembling  the  mother  and 
possessing  the  essential  features  of  B.  bp.  heteris. 

0693.203  :  Self- fertilized  seeds  of  3  guarded  specimens  belonging  to  the 
last-described  family  were  sown  together  October  12,  1906,  and  gave  a 
progeny  of  111  specimens,  84  of  which  were  typical  B.  bp.  heteris  and  27 
B.  bp.  tenuis,  or  in  the  ratio  3.1  :  1.     (See  plate  1.) 

As  no  other  artificially-produced  hybrids  between  biot5q:)es  of  B.  bursa- 
pastoris  have  been  studied  beyond  the  first  generation  and  only  facts  bear- 
ing upon  the  question  of  dominance  can  be  derived  from  these  cultures  as 
yet,  it  seems  best  to  postpone  their  discussion  until  the  second  generation 
has  been  studied.  This  is  the  more  important,  since  the  specimens  chosen 
for  crossing  in  a  number  of  instances  belonged  to  families  whose  relations 
to  the  4  biotypes  which  have  been  involved  in  the  above-described  natural 
hybrids  are  still  in  doubt. 

The  occurrence  of  supposed  hybrids  among  the  biotypes  of  Bursa  bursa- 
pastoris  has  been  noted  by  Almquist  (1907,  pp.  22  and  88-89),  who  also 
refers  to  descriptions  of  similar  cases  by  Von  Borbas  and  by  Grenier.  The 
actual  hybrid  character  of  the  plants  mentioned  is  very  doubtful,  however. 
The  assumption  of  their  hybridity  is  based  wholly  upon  the  facts  of  inter- 
mediacy  and  sterility,  together  with  the  vegetative  vigor  and  longevity 
which  are  certainly  correlated  with  sterility.  I  have  occasionally  observ^ed 
such  sterile  or  nearly  sterile  plants  in  progenies  produced  from  seeds  fully 


BIOTYPES  AND  HYBRIDS.  43 

guarded,  so  that  hybridity  was  excluded  with  such  care  as  is  possible. 
Intermediacy  in  the  size  of  flowers  is  not  at  all  strang-e  in  the  case  described 
by  Almquist,  for  the  culture  in  which  the  supposed  hybrids  occurred  was 
that  of  a  very  large-flowered  form.  Intermediacy  in  such  a  case  simply 
means  a  reduction  in  the  size  of  the  flowers.  In  all  of  my  hybrids  there 
has  been  no  apparent  decrease  in  the  number  or  viability  of  seeds  produced, 
and  it  will  be  recalled  that  there  is  only  one  clear  case  of  intermediacy, 
namely,  in  the  hybrids  between  B.  bp.  heteris  and  B.  bp.  rhojjtboidea,  in 
which  it  is  due  to  the  incomplete  dominance  of  the  former.  Perhaps  in 
other  cases  also  dominance  is  not  quite  complete,  but  it  is  so  nearly  so  that 
it  is  impossible  to  distinguish  certainly  between  the  heterozygote  and  one 
of  its  parents. 

HISTORY  OF  BURSA  HEEGERI. 

Eleven  years  ago  Professor  Heeger  found  some  specimens  of  a  crucifer- 
ous plant  growing  in  the  market-place  at  Landau,  Germany,  which  he  could 
not  identify.  In  general  habit  these  plants  resembled  the  almost  cosmo- 
politan species  Bursa  bursa-pastoris .  They  differed  from  the  latter  species, 
however,  in  having  the  seed-capsules  elliptical  in  longitudinal  section  and 
circular  in  cross-section  instead  of  flat  and  triangular  or  obcordate,  as  is 
characteristic  of  Bursa  bursa-pastoris . 

The  specimens  were  submitted  to  Solms-Laubach,  who  was  inclined  at 
first  to  refer  them  to  the  genus  Camelina,  which  is  characterized  by  nearly 
spherical  capsules,  and  Professor  Ascherson,  to  whom  he  showed  them,  was 
of  the  same  opinion.  Cultures  made  by  Solms-Laubach  from  seeds  secured 
from  Professor  Heeger  soon  indicated,  however,  the  near  relationship  of 
the  new  form  to  Bursa  bursa-pastoris,  when  in  1898  several  apparent  rever- 
sions to  the  capsule-form  of  B.  bursa-pastoris  were  noted.  Solms-Laubach 
(1900)  published  an  account  of  the  new  form,  assigning  to  it  the  name 
Capsella  kee^eri,  which  becomes,  according  to  the  rule  of  priority,  Bursa 
hee^eri  (Solms-Laubach). 

This  very  distinct  species  of  Bursa  has  attracted  considerable  attention, 
for  the  reason  that  its  occurrence  as  a  component  of  the  flora  of  a  region 
so  well  known  systematically  has  left  little  doubt  of  its  very  recent  origin 
from  B.  bursa-pastoris  by  mutation,  and  it  is  mentioned  by  De  Vries  (1901, 
pp.  477-478;  1905,  pp.  582-584)  as  an  instance  of  mutation  in  nature. 
Shortly  after  the  publication  of  the  original  account.  Bursa  heeger i  disap- 
peared from  the  type  locality  at  Landau,  owing  to  the  destruction  of  its 
habitat  by  covering  the  market-place  with  gravel,  and  it  has  been  reported 
from  nature  only  once  since  that  time,  though  it  has  been  widely  grown  in 
botanical  gardens. 

The  second  report  of  the  discovery  of  Bursa  heegeri  in  nature  was  made 
by  Laubert  (1905),  who  found  it  along  the  Dahlem  turnpike  in  1905,  but 


44  BURSA  BURSA-PASTORIS  AND  BUR.SA  HEEGERI  : 

here  the  Hkehhood  of  a  new  origfin  is  certainly  exceeded  by  the  probabihty 
that  a  seed  was  carried  to  this  spot  by  some  agency  from  a  nearby  culture, 
for  it  had  been  grown  for  several  years  at  Dahlem  in  an  unprotected  bed 
several  hundred  meters  from  the  place  in  which  Laubert  discovered  it. 
Hus  (1908)  takes  the  alternative  view,  however,  and  considers  this  a  case 
of  repeated  mutation.*  Laubert  points  out  that  in  addition  to  the  capsule 
character  noted  by  Solms-Laubach,  there  are  other  characters  of  the  stem 
and  inflorescence  which  serve  to  distinguish  B.  heegeri  from  its  supposed 
parent  and  which  would  suggest  a  more  distant  relationship  with  that  form 
than  had  been  supposed;  but,  on  the  other  hand,  he  found  that  in  both 
species  there  occur  frequent  instances  of  abnormal  pistillate  but  sterile 
flowers  in  the  lower  portion  of  the  flower-stem,  and  he  takes  this  fact, 
together  with  the  occurrence  of  capsules  occasionally  simulating  those  of 
B.  bursa-pastoris,  formed  when  B.  heegeri  is  attacked  by  Albugo  and  Pero- 
nospora,  as  additional  proofs  that  B.  heegeri  is  a  derivative  from  B.  bursa- 
pastoris.  Of  the  manner  of  its  origination  from  B.  bursa-pastoris  nothing 
is  known,  of  course,  but  Potonie  (1906)  suggests  that  it  is  a  reversion 
induced  by  some  pathological  condition. 

More  recently  Noll  (1907)  has  investigated  some  plants  resembling  B. 
heegeri,  which  had  already  been  found  by  Melsheimer  in  1882  in  himdreds 
in  a  field  of  Dattenberger  Flur  and  again  in  1884  on  a  height  at  Linz.  Mels- 
heimert  considered  these  plants  hybrids,  but  could  not  suggest  the  prob- 
able parents,  while  Komicke  and  Wirtgent  stated  that  they  are  doubtless 
identical  with  Bursa  heegeri.  Specimens  of  Melsheimer' s  plants  were 
placed  in  Petry's  herbarium  bearing  the  label  "Capsella  bursa-pastoris 
forma  caps,  ovatis. ' '  Noll  received  this  material  from  Petry,  together  with 
living  specimens  collected  by  the  latter  at  Didenhofen,  Metz,  Hagendin- 
gen,  and  Kreuznach.  A  careful  comparison  of  the  anatomical  features  of 
these  plants  with  those  of  Bursa  heegeri  and  B.  bursa-pastoris  led  Noll 
to  the  conclusion  that  the  Melsheimer  plants  are  not  Bursa  heegeri,  but 
a  sterile  form  of  B.  bursa-pastoris,  to  which  he  gives  the  name  Capsella 
pseudo-heegeri.  The  finding  of  these  plants  in  considerable  numbers  tends 
to  weaken  the  argument  that  the  discovery  of  B.  heegeri  in  a  region  so  well 
known  proves  it  to  be  a  recent  mutation.  Perhaps  Bursa  heegeri  will  yet 
be  discovered  in  some  abundance  in  some  locality  where  it  has  hitherto 
escaped  notice. 

*I  have  called  the  attention  of  Dr.  Hus  to  this  matter  and  he  concedes  in  a  letter 
that  my  explanation  of  the  occurrence  of  B.  heegeri  in  nature  at  Dahlem  is  probably  the 
correct  one. 

t  Mentioned  by  Noll,  but  not  verified  by  me. 


BIOTYPES  AND  HYBRIDS.  45 

HYBRIDS   BETWEEN   BURSA   BURSA-PASTORIS   AND 

BURSA    HEEGERI. 

The  prominent  part  which  mutation  may  have  taken  in  the  production 
of  new  species  makes  it  of  great  interest  to  know,  in  each  case,  just  what 
will  be  the  result  when  the  supposed  or  the  demonstrated  mvitant  is  self- 
fertilized  and  when  it  is  crossed  with  the  parental  form;  for  its  behavior  in 
these  two  cases  is  the  first  important  factor  in  determining  the  power  of 
the  mutant  to  maintain  itself  at  the  time  of  its  origin  and  its  capacity  to 
give  rise  to  a  successful  series  of  genetically  related  individuals  belonging 
to  an  independent  type. 

The  result  of  self -fertilization  was  investigated  by  Sol  ms-Laubach  before 
the  publication  of  his  original  account,  and  he  showed  that  the  characters 
that  differentiate  B.  hcegeri  from  B.  biirsa-pastoris  are  fully  heritable  in  a 
self -fertilized  line.  As  we  have  seen,  the  Bursas  normally  self -fertilize  to 
a  predominant  extent,  and  this  habit,  coupled  with  a  vigorous  constitution, 
would  seem  to  constitute  all  the  factors  necessary  to  successful  mainte- 
nance. I  have  now  determined  what  will  be  the  result  of  intercrossing  B. 
heegcri  and  B.  biirsa-pastoris. 

The  aspects  of  Bursa  heegeri  and  B .  bursa-pastoris  as  they  appeared  in 
my  cultures  were  so  different  that  at  the  first  I  was  skeptical  concerning 
the  near  relationship  which  has  been  assumed  to  exist  between  them. 
Bursa  heegeri  was  much  more  vigorous  than  B.  bursa-pastoris,  and  the  dif- 
ferences observed  in  the  inflorescence  by  Laubert  (1905)  were  strikingly 
apparent,  the  pedicels  of  the  capsules  being  shorter,  more  crowded  on  the 
rachis,  and  diverging  from  the  latter  at  a  wider  angle  (fig.  21).  While 
the  leaves  of  the  rosette  of  Bursa  heegeri  are  of  the  heteris  type,  having 
the  primary  and  the  rounded  secondary  lobes  readily  distinguishable,  the 
sinus  which  sets  off  the  latter  is  comparatively  shallow,  and  in  consequence 
the  secondary  lobe  appears  low  and  less  well-marked.  The  primary  lobe 
is  usually  broader  and  less  strongly  attenuate  than  in  B.  bursa-pastoris 
heteris.  Despite  these  considerable  differences,  however,  it  was  found  that 
B.  heegeri  may  be  crossed  with  B.  bursa-pastoris  with  perfect  ease  in  either 
direction  and  without  any  apparent  decrease  in  fertility,  though  I  crossed 
it  with  B.  bp.  simplex,  the  biotype  of  the  latter  species  which  is  most  unlike 
B.  heegeri. 

The  families  of  pure-bred  B.  heegeri  and  its  hybrids  with  B.  bp.  simplex 
may  be  briefly  considered  under  the  pedigree-numbers  used  during  their 
culture  : 

040.9  :  Seeds  of  Bursa  heegeri  received  through  Dr.  D.  T.  ]\IacDougal 
from  Professor  Solms-Laubach  were  sown  July  31,  1905,  and  produced 
26  plants,  all  of  which  agreed  with  the  above  description,  the  fluctuating 
variations  being  extremely  slight.     The  unguarded  seeds  of  one  of  these 


46 


BURSA  BURSA-PASTORI.S  AND  BUR.SA  HEEGERI 


Fig.  21. — Bursa  heegeri  (Solms-Laubach).     From  photographs  taken 
at  three  different  stages  of  development. 


BIOTYPES  AND  HYBRIDS.  47 

(059.56)  were  sown  November  1,  1906,  and  produced  24  plants,  all  agree- 
ing- perfectly  with  the  original  type  as  described  (fig.  22).  These  became 
diseased  later,  however,  and  produced  no  seed. 

056.88  :  This  plant  was  considered  a  typical  specimen  of  B.  bp.  simplex, 
though  a  little  more  vigorous  and  broader-leafed  than  usual.  It  was  care- 
fully castrated  and  pollinated  with  pollen  from  a  i)lant  belonging  to  my 
first  culture  of  B.  heegeri  (059).     The  seeds  were  sown  April  25,  1906,  and 


Fig.  22. — Bursa  heegeri.     Second  controlled  generation. 

produced  108  offspring,  all  resembling  B.  heegeri  more  closely  than  B.  hp. 
simplex,  but  they  differed  markedly  from  the  former  because  of  the  imper- 
fect-dominance of  the  heteris  characteristics.  On  this  account  these  plants 
had  some  of  the  characteristics  of  B.  bp.  rhomboidea  and  could  be  properly 
described  as  intermediate  between  B.  bp.  rhomboidea  and  B.  bp.  heteris  (fig. 
23).  A  few  of  these  died  without  seeding,  but  all  that  came  to  maturity 
had  the  triangular  capsules  typical  of  Bursa  bursa-pastoris .  One  family 
was  raised  from  unguarded  seeds  of  one  of  these  plants  (0688.212),  as 
described  below. 


48 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 


059.89  :  This  typical  specimen  of  Bursa  heegeri^zs,  castrated  and  polli- 
nated with  pollen  from  a  specimen  of  B.  bp.  simplex  which  differed  from 
the  usual  condition  of  that  biotype  in  having  the  apex  of  the  leaves  taper- 
ing- and  acutish.  This  condition  has  been  shown  to  be  merely  a  fluctua- 
tion (see  p.  26),  so  that  the  plant  used  in  this  cross  is  to  be  considered  a 


Fig.  23. — Bursa  bursa-pastoris  simplex  X  heegeri,  Fp  Incomplete  domi- 
nance of  the  heteris  characteristics  produces  a  form  resembling  B.  bp. 
rhomboidea,  but  mostly  with  longer,  sharper  primary  lobes. 

typical  specimen  of  B.  bp.  simplex.  The  pollen-parent  of  this  family  was 
a  sib  of  the  pistil-parent  of  the  family  last  described,  and  the  two  speci- 
mens of  B.  heegeri  involved  in  these  two  families  were  likewise  sibs,  so 
that  these  two  crosses  were  essentially  reciprocal.  The  seeds  were  sown 
April  25,  1906,  and  produced  23  offspring,  which  were  intermediate  be- 
tween B.  bp.  heteris  and  B.  bp.  rhomboidea  as  to  the  rosette  characters,  and 
in  which  they  were  indistinguishable  from  the  reciprocal  hybrid  family, 


BIOTYPES  AND  HYBRIDS.  49 

0688.  Most  of  these  plants  produced  fruit,  and  in  all  cases  this  was  flat 
and  triang-ular  to  obcordate,  like  that  of  B.  biirsa-pastoris .  Ung-uarded 
seeds  of  3  of  these  and  guarded  seeds  of  1  were  used  for  the  production  of 
second-g-eneration  families,  1  as  0689.196  and  3  collected  tog-ether  as 
0689.197.     The  descriptions  of  the  second-generation  families  follow: 

0889. 198  :  Seeds  of  this  plant,  which  had  been  carefully  gfuarded  against 
cross-pollination,  were  sown  October  12,  1906,  and  217  plants  were  raised, 
of  which  188  lived  to  produce  seed.  These  had  the  following  composition: 
98  were  B.  bnrsa-pastoris  keteris,  36  B.  bp.  tenuis,  2>2  B.  hp.  rhomboidea,  13 
B.  bp.  simplex,  5  B.  heegeri  heteris,  1  B.  h.  tenuis,  2  B.  h.  rhomboidea,  and 
\  B.  h.  simplex,  giving,  so  far  as  the  rosette-characters  are  concerned,  a 
very  close  agreement  with  the  ratio  9:3:3:1,  but  in  the  form  of  capsule 
showing  a  very  great  preponderance  of  the  bursa-pastoris  type. 

0689.197:  vSeeds  of  3  unguarded  sibs  of  the  parent  of  the  last  family 
were  sown  October  15,  1906,  under  this  number.  Of  2,014  offspring,  1,815 
came  to  maturity,  and  were  recorded  as  having  the  following  composition  : 
1,032  were  B.  bursa-pastoris  Jieteris,  331  B.  bp.  tenuis,  2>02B.  bp.  rhomboidea , 
78  B.  bp.  simplex,  45  B.  heegeri  heteris,  13  B.  h.  tenuis,  13  B.  h.  rhomboidea, 
and  1  B.  h.  simplex  (plates  3  and  4).  Again  there  is  a  close  agreement  in 
the  leaf -characters  with  the  typical  dihybrid  ratio,  9:3:3:1,  and  a  notable 
deficiency  in  the  occurrence  of  the  heegeri  type  of  capsule. 

0688.212  :  Seeds  of  this  unguarded  plant,  which  was  a  reciprocal  of  the 
parents  of  the  last  two  families  described,  were  sown  October  12,  1906,  and 
produced  a  large  progeny,  of  which  621  unselected  plants  were  potted  for 
study  and  the  rest  discarded.  Of  these  621  plants,  537  reached  maturity 
and  were  classified  thus  :  317  B.  bursa-pastoris  heteris,  102  B.  bp.  tenuis, 
61  B.bp.  rhomboidea,  21  B.  bp.  simplex,  19  B.  heegeri  heteris,  7  B.  h.  tenuis, 
and  4  /?.  //.  rhomboidea,  no  B.  h.  simplexheing  observed.  The  same  general 
relations  of  the  rosette-characters  and  capsule -characters  are  obvious  here 
as  appeared  in  the  other  two  Fj  families  described,  but  there  is  not  quite 
as  close  agreement  with  the  ratio  9  :  3  :  3  :  1  as  in  the  other  families,  prob- 
ably because  this  family  became  somewhat  diseased  and  the  distinguishing 
of  the  several  types  of  rosette  became  consequently  more  difficult. 

Reviewing  the  results  of  crossing  Biirsa  bursa-pastoris  simplex  and  B. 
heegeri,  it  is  seen  that  the  Fi  hybrids  are  essentially  uniform,  no  matter  in 
which  direction  the  cross  is  made,  and  that  the  rosette  in  either  case  is  of 
the  heteris-rhomboidea  type,  owing  to  the  incomplete  dominance  of  heteris, 
while  the  capsule  is  always  of  the  bursa-pastoris  type.  In  Fa  there  appear 
the  4  types  of  rosette  already  described,  in  combination  with  each  tyj^e 
of  capsule.  The  rosette  presented  many  instances  of  the  best-developed 
examples  of  the  4  described  forms,  particularly  interesting  being  the  fact 


50 


BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI 


that  much  better-developed  hetcris  rosettes  occur  in  the  Fj  than  are  seen  in 
pure-bred  B.  heegeri,  though  these  heteris  characteristics  must  have  come 
directly  from  the  heegeri  side  of  the  cross. 

The  numerical  results  of  these  crosses  may  be  tabulated  thus  : 

Btirsa  bp.  simplex  {abC)  X  Bursa  h.  heteris  (ABc) 
(056)  (059) 

Bursa  bp.  heieris-rhomboidea  {ABCabc) 
(0688  and  0689) 


Bursa-pastoris  series  (C): 

heteris 

tenuis 

rhomboidea 

simplex 

Heegeri  stv'xts  (c): 

heteris .'.... 

tenuis 

rhomboidea 

simplex 


06196 


98 
36 
32 
13 

5 
I 

2 

I 


19.9: 


06197 


1032 

331 

302 

78 

45 

13 

13 

I 


24.2 


06212 


Total. 


317 
102 

67 
21 

19 
7 
4 
o 


1447 
469 
401 
112 

69 
21 

2 


Expected. 


1368 
456 

456 
152 

63 
21 

21 
7 


16.9:  I         21.9:   I 


3-0 


I 


In  this  table  the  pedigfree-numbers  06196  and  06197  represent  families 
in  which  B.  heegeri  was  the  mother,  while  06212  is  the  reciprocal  cross. 
It  will  be  seen  that  there  is  no  essential  difference  between  these  two 
crosses.  In  the  rosette-characters  these  numbers  are  very  close  to  the 
Mendelian  ratios,  amongf  both  those  having  bursa-pastoris  capsules  and 
those  having  heegeri  capsules,  showing  b}^  these  facts  that  the  heegejd 
rosette  has  the  same  allelomorphic  composition  as  the  rosette  of  B.  bursa- 
pastoris  heteris,  and  that  these  rosette-characters  are  not  coupled  in  any 
but  a  chance  way  with  the  form  of  the  capsule.  This  independence  of 
characters  is  thus  a  matter  of  great  importance  in  the  production  of  new 
elementary  forms ;  for  before  this  cross  was  made  there  existed,  so  far  as 
is  known,  but  one  elementary  species  of  B.  heegeri,  while  otit  of  the  cross 
came  4  distinct  elementary  forms  of  this  species. 

While  there  is  a  perfect  agreement  with  Mendelian  ratios  in  rosette- 
characters,  the  capsules  give  a  surprising  departure.  The  capsule-form  is 
perfectly  alternative;  there  has  never  appeared  any  intermediate  condition 
in  the  plants  in  my  cultures,  and  the  "reversions"  observed  by  Solms- 
Laubach  and  Laubert  were  pathological.  As  the  bursa-pastoris  type  is 
dominant,  simple  Mendelian  expectation  would  require  the  appearance  of 
1  B.  heegeri  in  every  4  Fj  individuals.  Out  of  2,540  plants  of  the  F2  gen- 
eration observed,  only  111  were  B.  heegeri,  or  approximately  1  in  23.  That 
this  result  should  be  consistently  given  in  3  different  pedigrees  represent- 
ing reciprocal  crosses  adds  greatly  to  the  weight  that  is  to  be  attached  to 


BIOTYPES  AND  HYBRIDS.  51 

it.  The  greatest  frequency  in  any  pedigree  was  1  in  nearly  18,  and  the 
least  frequency  was  1  in  25.* 

I  take  the  fact  that  the  reciprocal  crosses  give  similar  ratios  to  indicate 
that  the  heegeri  type  of  capsule  is  dependent  upon  something  carried  by  the 
germ-cells,  and  can  not  be  a  pathological  condition  transmissible  from 
mother  to  offspring  somatically,  a  possibility  which  might  account  for 
apparent  heritability  of  characters  in  a  self -fertilized  line,  but  could  not 
well  account  for  equal  results  in  reciprocal  crosses.  Normal  Mendelian 
phenomena  are  believed  to  rest  pretty  securely  on  the  method  of  formation 
of  the  chromosomes  during-  the  reduction  division,  but  no  scheme  of 
behavior  occurs  to  ine  which  would  result  in  the  i:)roduction  of  a  heegeri 
homozygote  in  only  1  individual  in  23. 

It  is  conceivable  that  the  union  of  heegeri  germ-cells  in  the  hybrids  forms 
a  less  successful  combination  than  that  into  which  the  B.  biirsa-pastoris 
determiner  enters,  and  that  therefore  fewer  successful  zygotes  are  formed 
by  such  unions,  and  it  is  also  conceivable  that  only  a  small  percentage  of 
the  B.  /?(?£'^^r/ succeeded  in  reaching  sexual  maturity,  since  in  each  of  these 
families  a  considerable  number  of  individuals  failed  to  fruit,  but  the  assump- 
tion that  every  individual  that  failed  to  seed  was  a  B.  heegeri  would  not 
nearly  bring  that  form  up  to  one-fourth  of  the  entire  progeny.  There  was 
no  evidence  in  my  cultures  that  B.  heegeri  is  in  any  way  inferior  to  B. 
bursa-pastoris  or  that  it  is  any  less  likely  to  mature;  neither  have  I  observed 
any  indication  of  the  material  lessening  of  fertility  which  would  obtain  if 
almost  all  of  the  heegeri  homozygotes  should  fail  in  the  initial  stages  of 
development. 


*Dr.  Correns  tells  me  that  he  also  made  the  cross  between  Bursa  bitrsa-pastoris  and 
B.  heegeri  several  years  ago  and  likewise  found  a  deficiency  in  the  number  of  specimens 
having  the  heegeri  type  of  capsule,  but  as  his  numbers  were  small  he  considered  the 
deficiency  due  alone  to  the  inadequate  numbers. 


52  BUR.SA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

DISCUSSION  OF  RESULTS. 

The  occurrence  of  elementarj^  species  within  the  limits  of  recognized 
systematic  or  Linnean  species  is  undoubtedly  very  general,  as  De  Vries 
has  maintained,  and  much  of  the  ordinary  conception  as  to  the  variability 
of  certain  species  is  attributable  to  this  fact.  Thus  in  the  specific  case  with 
which  we  are  dealing-  here.  Bursa  is  recog'nized  by  all  taxonomists  as 
exceedingly  variable,  but  each  single  biotype  of  Bursa  is  much  less  vari- 
able than  the  Linnean  species  taken  as  a  whole,  for  when  grown  under 
favorable  conditions  there  are  certain  characteristics  which  are  found  in 
every  individual  of  the  given  biotype  which  are  not  present  in  the  members 
of  other  biotypes. 

All  the  apparent  evidence  for  the  permanent  change  of  species  through 
selection  finds  a  ready  explanation  on  the  assumption  that  the  selection 
has  merely  eliminated  certain  biotypes  from  the  original  mixture  with 
which  the  selection  started,  thus  leaving  the  theory  that  fluctuations  are 
inherited  or  are  capable  of  leading  either  directly  or  indirectly  to  the  modi- 
fication of  any  biotype  wholly  tinsupported.  This  is  not  to  say,  of  course, 
that  such  modification  is  impossible  or  that  it  does  not  take  place,  but  merely 
that  such  a  proposition  must  rest  ^^pon  experience  still  to  be  gained. 

There  is  some  variation  among  the  members  of  the  single  biotype.  This 
variation  is  of  the  fluctuating  kind  by  defiiiition.  The  most  usual  varia- 
tions of  this  kind  are  those  which  result  from  crowding,  shading,  poor  soil, 
drought,  the  attacks  of  insects,  or  other  conditions  which  decrease  the  vigor 
of  the  plants,  the  effect  being  to  arrest  differentiation  in  more  or  less  juve- 
nile stages  of  development.  This  feature  has  been  very  troublesome  at 
times  in  my  cultures,  since  it  is  impossible  under  such  circumstances  to 
determine  by  inspection  to  which  biotype  a  given  specimen  belongs.  The 
breeding-test  is  the  only  method  by  which  such  a  determination  can  be 
made  and  when  many  specimens  have  their  distinguishing  characteristics 
rendered  latent  in  this  way  the  labor,  time,  and  patience  required  for  com- 
plete classification  become  unduly  increased. 

This  suppression  of  characteristics  through  fluctuation  also  stands  in  the 
way  of  the  classification  of  specimens  observed  in  nature  in  various  habi- 
tats where  they  have  gTown  under  different  conditions  which  are  in  a  large 
degree  unknown,  and  on  this  accotint  there  can  be  no  question  as  to  the 
advisability  of  retaining  the  Linnean  names  in  practical  taxonomy  for  the 
designation  of  such  complex  groups  of  biotypes.  Workers  in  all  other 
botanical  fields  inust  bear  in  mind,  however,  that  conclusions  reached  with 
one  biotype  inay  not  hold  in  some  other  biotype  of  the  same  Linnean  species. 

The  demonstration  of  the  elementarj-  character  of  these  biotypes  of  Bursa 
is  made  complete  by  the  fact  that  they  Mendelize  on  being  crossed,  for  such 


BIOTYPES  AND  HYBRIDS.  53 

behavior  can  only  rest  upon  the  presence  of  an  internal  factor,  of  whatever 
nature,  capable  of  independent  movement  at  the  time  of  the  reduction- 
division  in  the  formation  of  the  gferm-cells.  It  may  be  assumed,  perhaps, 
that  this  factor  or  determiner  is  incapable  of  division  at  the  time  of 
the  reduction-division  ;  but  however  it  is  explained,  the  result  is  the  pro- 
duction of  certain  individuals  (homozyg'otes)  which  produce  g-crm-cclls  all 
of  one  kind  with  respect  to  a  particular  character  and  which  must  there- 
fore breed  true  with  respect  to  that  character,  and  other  individuals  (het- 
erozyg"otes)  which  produce  germ-cells  of  two  kinds  with  respect  to  the  same 
character  and  which  consequently  can  not  breed  true.  Individuals  which 
possess  the  same  characteristics  and  which  arc  homozyg"ous  with  respect 
to  all  of  these  characteristics,  collectively  form  a  biotype  ;  hence  the  state- 
ment that  the  Mendelian  inheritance  of  the  several  forms  of  Bursa  demon- 
strate beyond  a  possible  question  that  they  are  distinct  biotypes.  I  will 
not  be  understood,  however,  to  imply  that  biotypes  must  Mendelize  on 
being-  crossed,  for  many  are  known  which  do  not,  as,  for  example,  most  of 
the  various  biotypes  of  Oenothera. 

The  importance  of  Mendel's  discoveries  for  our  conception  of  the  signifi- 
cance of  different  kinds  of  variations  in  the  evolution  of  plants  and  animals 
is  now  generally  recognized.  It  is  seen  that  in  the  presence  of  Mendelian 
inheritance  the  ' '  swamping  "  of  a  new  character  by  crossing  with  the 
parent  form  does  not  take  place,  and  that  the  very  kind  of  variation  which 
was  supposed  to  be  swamped  by  crossing  is  just  the  kind  which  is  most 
certain  to  be  preserved.  The  new  form  may  completely  disappear  when 
crossed,  but  it  is  only  hidden  momentarily,  not  destroyed.  Thus  when 
Bursa  /leegeri  Qvoss&s  with  its  parent  Bursa  bursa-pastoris,  all  of  the  offspring 
are  indistingniishable  from  the  latter,  but  in  the  next  generation  a  portion 
of  the  offspring  are  just  as  typical  B.  heegeri  as  the  original  specimen,  and 
no  transitional  forms  occur.  If  the  dominance  of  the  triangular  capsules 
were  incomplete  there  would  be  some  transitional  stages,  but  this  would 
not  modify  the  situation  in  any  essential  manner,  as  there  would  still  be 
just  as  many  typical  homozygotes  as  before  which  would  breed  true  when- 
ever they  chanced  to  be  fertilized  by  their  own  kind,  and  if  fertilized  again 
by  the  parent  form  they  would  form  heterozygotes  which  would  produce 
as  large  proportion  of  typical  B.  heegeri  as  was  produced  by  the  heterozy- 
gotes of  the  previous  generations. 

I  have  shown  elsewhere  (Shull,  1907«)  that  this  capacity  of  a  mutant 
to  disappear  upon  crossing  with  its  parent  may  be  a  great  advantage  in  the 
struggle  for  existence  under  certain  conditions,  and  if  we  grant  that  evolu- 
tion is  in  the  main  retrogressive  (Shull,  1907<^),  the  new  forms  will  be 
generally  recessive  to  the  parent  and  will  thus  be  in  position  to  profit  by 
whatever  advantage  recessiveness  gives.  From  the  data  I  have  as  to  the 
geographic   distribution    of   the    several    biotypes  of   B.  bursa-pastoris  it 


54  BURSA  BURSA-PASTORIS  AND  BURSA  HEEGERI  : 

appears  that  B.  bp.  heteris  is  much  more  grenerally  distributed  than  any  of 
the  others ,  and  this  favors  the  view  that  this  is  the  original  type  from  which 
B.  bp.  rJiomboidea^  tenuis,  and  simplex  ^nqxq,  derived  by  retrog-ressive  muta- 
tion. Recessiveness  of  the  last  three  to  the  first  is  probably  rarely  if  ever 
any  advantage  to  them,  however,  since  all  of  these  forms  appear  to  be 
about  equally  adapted  to  the  range  of  habitats  in  which  they  grow. 

Mendelian  inheritance  also  has  an  important  bearing  upon  the  distribu- 
tion of  the  various  biotypes  of  a  species;  for  the  transportation  of  a  single 
hybrid  seed  may  carr}^  all  of  the  biotypes  which  are  related  to  one  another 
in  the  Mendelian  way,  however  many  there  may  be,*  and  two  pure-bred 
seeds  landed  in  the  same  vicinity  may  lead  to  the  same  result.  Thus  any 
seed  of  B.  bp.  heteris  which  has  been  produced  by  pollination  with  pollen 
from  B.  bp.  simplex,  or  vice  versa,  or  any  seed  of  B.  bp.  rhomboidea  that 
has  resulted  from  pollination  with  pollen  from  B.  bp.  temiis,  or  vice  versa, 
will  give  rise  to  a  progeny  in  the  F2  which  will  include  all  four  of  these 
biotypes,  and  self -evidently  two  pure-bred  seeds  representing  either  of 
these  two  pairs  of  elementary  species  will  carry  the  capacity  to  produce  the 
same  four  types  in  the  third  generation  from  the  time  they  find  themselves 
in  juxtaposition  in  a  new  locality. 

The  same  principle  would  hold  if  there  were  3  pairs  of  Mendelian  char- 
acters involved,  but  then  8  biotypes  might  be  carried  by  a  single  hybrid  seed. 
This  situation  would  be  realized  by  the  material  dealt  with  in  this  paper 
if  a  single  seed  of  B.  bp.  simplex  pollinated  by  B.  heegeri,  or  vice  versa, 
were  taken  to  a  new  locality.  Transcau  (1907)  has  pointed  out  how  on 
the  same  principle  any  number  of  biotypes  might  be  introduced  by  means 
of  a  single  pollen  grain  into  a  new  locality  where  a  single  biotype  had 
existed  before,  and  to  which  heavy  seeds  might  find  much  greater  diffi- 
culty in  being  transported. 

Still  another  important  effect  of  Mendelian  inheritance  in  the  promotion 
of  organic  evolution  is  brought  out  by  my  crosses  between  B.  bursa-pastoris 
and  B.  heegeri,  namely,  the  production  of  parallel  series  of  biotypes  in 
nearly  related  species.  Up  to  the  time  this  cross  was  made  B.  heegeri  y^^s 
known  only  in  the  heteris  form,  but  among  the  hybrid  offspring  were  4 
distinct  pure-breeding  biotypes  of  B.  heegeri.  It  is  thus  seen  that  the 
single  mutation  by  which  B.  heegeri  originated  from  B.  bursa-pastoris 
doubled  the  number  of  possible  biotypes  of  Bursa  in  the  world,  provided 
all  such  other  biotypes  behave  as  do  the  four  under  discussion  in  this  paper. 

It  may  be  added  that  the  facts  here  shown  that  the  rosette  of  B.  heegeri 
represents  the  same  Mendelian  units  that  are  present  in  B.  bursa-pastoris 
and  that  there  is  only  a  single  fundamental  difference  between  these  two 
species  is  the  best  possible  proof  of  the  origin  of  B.  heegeri  from  B.  bursa- 

*Except  in  the  presence  of  "spurious  allelomorphism." 


BIOTYPES  AND  HYBRIDS.  55 

pastoris,  despite  the  considerable  differences  of  general  aspect  both  of  foliag-e 
and  of  inflorescence.  It  is  possible  that  these  differences  of  aspect  may 
rest  upon  the  presence  of  still  other  units  which  have  not  been  taken  into 
account  in  these  studies,  but  if  such  should  prove  to  be  the  case,  the  gen- 
eral conclusions  must  be  the  same. 

I  wish  to  acknowledge  my  appreciation  of  the  facilities  which  have  been 
placed  at  my  disposal  by  the  Station  for  Experimental  Evolution  of  the 
Carnegie  Instittition  of  Washington,  and  of  the  faithfulness  of  my  assistants, 
who  have  greatly  aided  me  with  the  technical  side  of  this  investigation. 
My  thanks  are  due  to  my  brother,  J.  Marion  Shull,  dendrological  artist  of 
the  United  States  Forest  Service,  for  the  drawings  reproduced  in  figs.  1 
to  23.  I  particularly  desire  to  acknowledge  my  indebtedness  to  Dr.  E.N. 
Transeau,  in  whose  charge  the  cultures  of  B.  bursa-pastoris  X  heegeri  were 
left  during  my  absence  in  California  in  the  spring  of  1907.  Much  of 
the  work  of  grouping  the  F,  into  the  appropriate  classes  was  done  by  him. 

SUMMARY. 

(1)  Bursa  {Capsella)  biirsa-pastoris  is  a  composite  species  made  up  of 
at  least  4  and  possibly  many  distinct  elementary  species  or  biotypes  ; 
4  of  these  are  described  tmder  the  names  Bursa  bursa-pastoris  heteris, 
B.  bp.  tetiuis,  B.  bp.  rhomboidea,  and  B.  bp.  simplex.  Except  for  the  sup- 
pression of  characteristics  due  to  bad  treatment,  the  fluctuations  of  these 
forms  are  slight. 

(2)  These  4  biotypes  cross  readily,  giving  in  each  case  a  uniform 
Fi  and  typical  Mendelian  splitting  in  Fj .  They  are  differentiated  from 
each  other  by  2  unit-characters,  namely,  elongated  primary  lobes  of  the 
climax-leaves  and  a  rounded  secondary  lobe  in  the  distal  axil  of  the  primary 
lobes.  When  B.  bp.  heteris  is  crossed  with  B.  bp.  simplex,  and  when  B. 
bp.  tenuis  is  crossed  with  B.  bp.  rJwmboidea,  the  Fi  is  intermediate  between 
B.  bp.  heteris  and  B.  bp.  rhomboidea  because  of  the  imperfect  dominance  of 
B.  bp.  heteris,  and  the  Fj  consists  of  heteris,  tenuis,  rhomboidea,  and  simplex 
in  the  ratio  9:3:3:1. 

(3)  The  sterile  or  nearly  sterile  plants  supposed  by  Almquist  to  be 
hybrids  were  probably  not  hybrids,  as  similar  forms  were  noted  in  guarded 
cultures,  and  in  all  of  my  hybrids  no  decrease  in  fertility  was  apparent. 

(4)  Bursa  heegeri  differs  from  B.  bursa-pastoris  in  the  aspect  of  its 
rosette  and  inflorescence,  but  most  notably  in  the  form  of  the  capsule.  Its 
climax-leaves  are  of  the  same  general  type  as  those  of  B.  bp.  heteris. 

(5)  Bursa  heegeri  may  be  readily  crossed  with  B.  bursa-pastoris.  When 
crossed  with  B.  bp.  simplex,  an  Fi  is  obtained  which  more  nearly  resem- 
bles B.  heegeri  in  rosette  characters,  but  has  the  capsule-form  of  B.  bursa- 
pastoris.     In  F,  there  appear  the  4  forms  of  rosette  already  mentioned,  in 


56  BURSA  BURSA -PASTORIS  AND  BURSA  HEEGERI  : 

the  ratio  9:3:3:1,  in  association  with  each  type  of  capsule.     The  ratio 
of  the  bursa-pastoris  type  of  capsule  to  the  heegeri  type  is  about  22  :  1. 

(6)  Bursa  heegeri  possesses  the  same  unit -characteristics  in  the  rosette 
as  B.  hp.  /leteris,  which  serves  to  further  confinn  its  direct  derivation  from 
that  species. 

(7)  The  fact  that  the  various  forms  of  Bursa  show  Mendelian  inherit- 
ance on  crossing'  is  conclusive  proof  that  they  are  distinct  biotypes,  not- 
withstanding: the  fact  that  their  disting^uishing  characteristics  are  readily 
rendered  latent  by  fluctuation. 

(8)  In  the  presence  of  Mendelian  inheritance  the  swamping  of  a  new 
characteristic  does  not  take  place,  and  the  kind  of  variation  which  has  been 
supposed  to  be  swamped  by  crossing'  is  just  the  kind  that  is  most  certain  to 
be  preserved. 

(9)  The  recessiveness  of  a  newly  arisen  form  is  to  be  considered  an 
advantage  in  its  struggle  with  the  parent,  whenever  the  former  is  in  any 
way  less  adapted  to  its  environment  than  the  latter.  This  principle  appar- 
ently has  no  bearing  upon  these  biotypes  of  Bursa,  however,  as  all  appear 
to  be  equally  adapted  to  the  range  of  habitats  in  which  they  live. 

(10)  Bursa  bursa-pastoris  hetcris  appears  to  have  a  more  general  distri- 
bution than  any  of  the  other  forms.  This  is  probably  the  primitive  type 
from  which  the  other  biotypes  have  been  derived  by  retrogressive  mutation. 

(11)  Mendelian  inheritance  also  assists  in  the  distribution  of  the  vari- 
ous biotypes,  since  a  single  hybrid  seed  or  two  pure-bred  seeds  may  carry 
into  a  new  locality  as  many  distinct  biotypes  as  are  related  to  each  other 
by  Mendelian  characters.  A  single  pollen-grain  may  do  the  same  provided 
one  biotype  is  already  present  in  the  locality  in  question. 

(12)  Crosses  between  nearly  related  species  may  g'ive  rise  to  parallel 
series  of  biotypes  in  the  two  species  concerned.  Before  my  crosses  between 
Bursa  heegeri  and  B.  bursa-pastoris  were  made,  the  former  was  known  only 
in  1  form,  but  out  of  the  crosses  came  4  distinct  biotypes  of  B.  heegeri, 
corresponding  with  the  4  biotypes  which  have  been  demonstrated  in  B. 
b  u  rsa-pastoris . 

(13)  Each  mutation  which  results  in  the  appearance  of  a  new  Mende- 
lian unit-character  doubles  the  possible  number  of  Mendelian  biotypes 
belonging'  to  the  species  in  question,  however  numerous  they  may  be 
already,  except  as  limited  by  "spurious  allelomorphism." 

Station  for  Experimental  Evolution,  July  11,  1908. 


BIOTYPES  AND  HYBRIDS.  57 

LITERATURE  CITED. 

AlxMquist,  £. 

1907.  Studien    iiber   die    Capsella   biirsapasioris  ( L.).     Acta   Horti   Bergiani,   4: 

No.  6,  pp.  92,  figs   66,  May  15,  1907. 
De  Vries,  H. 

1901.     Die  Mutationstheorie.     I.  Die  Entstehung  der  Arten  durcli  Mutation,     pp. 

xii  +  64S,  pis.  8,  figs.  181,  1901.     Leipzig.     (See  pp.  477-478.) 
1905.     Species   and    varieties:  their    origin    by    mutation,     pp.     xviii  +847,     1905. 
Chicago.     (See  pp.  582-5S4.) 
Hus,  H. 

1908.  Fasciations  of  known  causation.     Amer.  Nat.,  42:  81-97,  2  figs.,  Feb.,  1908. 
Laubert,  R. 

1905.  Notizen  iiber  Capsella  Jieegeri  Solms.     Verh.  Bot.  Vereins  Provinz  Branden- 

burg, 47:   197-199,  4  figs.,  1905- 

LOTSY,  J.  p. 

1906.  Vorlesungen  iiber  Deszendenztheorien  mit  besonderer  Beriicksichtigung  der 

botanischen  Seite  der  Frage.        Erster  Teil,  pp.  384,  pis.  2,  figs.  124,  1906, 
Jena.     (See  pp.  179-181.) 
Noll,  F. 

1907.  Ueber  eine  Heegeri-ahnliche  Form  der  Capsella  bursa-pastoris  Moench.  Sitz- 

ungsber.  Niederrhein.  Gesells.  Nat.-u.  Heilkunde,  5  pp.,  1907,  Bonn. 

POTONIE,  H. 

1906.  Capsella  hee^eri  eine  pathologische  Erscheinung  mit  atavistischen  Momen- 

ten?     Naturwis.  Wochenschr.,  21  (n.  s.,  5):  788-791,  2  figs.,    Dec.  9,  1906. 
Shull,  G.  H. 

1907a.  Elementary  species  and  hybrids  of  Bursa.    Science,  n.  s.,  25:  590-591,  Apr. 

12,  1907. 
i()o-jb.     The  significance  of  latent  characters.     Science,  n.  s.,  25:  792-794,  May  17, 
1907. 

1908.  A  new  Mendelian  ratio  and  several  types  of  latency.     Amer.  Nat.,  42:  433- 

451,  July,  190S. 
Solms-Laubach,  H.  j  tt 

1900.     Cruciferienstudien.     L     Capsella  heegeri  ^o\m?,   erne    neuentstandene  term 

der  deutschen  Flora.     Bot.  Zeit.,  58:  167-190,  pi.  vii,  1900. 
Transeau   E.  N. 

1907.  Hybridization   a  factor  in  migration  and  competition.      Science,  n.  s.,  25: 

269-270,  Feb.  15, 1907. 


Shull 


Plate    I 


Fig.    I.  —  Bursa   bursa-pastoris   heteris.     Dominant  form  from  the  F2   of  an  artificial  cross  of  B.  bp. 
heteris  X  tenuis.     The  grandparents  were  sibs  of  the  plants  shown  in  text-figures  4  and  9. 

Fig.  2.  —  Bursa   bursa-pastoris   tenuis.      Recessive  form  from  the  same  family  as  the  plant  shown  in 
figure   1 . 


Shull 


Plate  2 


^^p^       06130 


06130 


F'g-  I-  —  Bursa  bursa-pastoris  rhomboidea.  Dominant  form  in  the  F2  of  a  liybrid  family  repre- 
senting the  natural  cross,  B.  bp.  simplex  X  rhomboidea.  The  parent  was  a  sib  of  the  plant 
shown  in  text-figure  20  and  had  the  same  characteristics. 

Fig.  2.  —  Bursa  bursa-pastoris  simplex.  Recessive  form  in  the  same  hybrid  family  whose  dominant 
form  is  represented  by  figure   1. 


» 


Shull 


Plate  3 


06197 


Fig.    1.     Bursa  heegeri  ><  bursa- pasloris  simplex.     A  heteris  rosette  in  the  F->. 
Fig.  2.     Bursa  heegeri  X  bursa- pastoris  simplex.     A  tenuis  rosette  in  the  F2. 


Shull 


Plate  4 


06197 


Fig.    I.     Bursa  heegeri  X  bursa-pastoris  simplex.     A  rhomboidea  rosette  in  the  F-... 
Fig.  2.     Bursa  heegeri  X  bursa-pastoris  simplex.     A  simplex  rosette  in  the  F2. 


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